Simple Slow Cooker French Onion Soup

Here is my favorite slow cooker French onion soup recipe, with sweet caramelized Vidalia onions, rich beef broth, fresh thyme, and topped with toasted baguette slices and melted Gruyère cheese.

This soup is perfect for those busy weeknights when you want something warm and comforting but don’t have time to stand over the stove. I just throw everything in the slow cooker in the morning, and by dinner time, the whole house smells amazing and everyone’s ready to eat.

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What Kind of Onions Should I Use?

Vidalia onions are called for in this recipe because they’re naturally sweet and become even sweeter as they cook down slowly, which is perfect for French onion soup. If you can’t find Vidalias at your store, other sweet onion varieties like Walla Walla or Maui onions will work just as well. Yellow onions are a solid backup option too – they’re available year-round and will give you that classic French onion soup flavor, though they’re slightly less sweet than Vidalias. Whatever you choose, make sure to slice them thinly and evenly so they cook at the same rate in your slow cooker.

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Options for Substitutions

This classic soup is pretty forgiving when it comes to swaps, so here are some options if you need them:

• Vidalia onions: Sweet Vidalia onions are ideal, but you can use yellow onions, white onions, or even a mix of different onions. Red onions work too, though they’ll give the soup a slightly different color and a bit more bite.
• Gruyère cheese: Gruyère is traditional and melts beautifully, but Swiss cheese, Emmental, or even a combination of mozzarella and Parmesan will work if that’s what you have. Just avoid pre-shredded cheese if possible – it doesn’t melt as smoothly.
• Beef broth: For a different flavor profile, try using a combination of beef and chicken broth, or go fully vegetarian with vegetable broth. The soup will be lighter but still tasty.
• Sherry vinegar: Red wine vinegar or white wine vinegar can step in for sherry vinegar. You can also use a splash of dry sherry or white wine if you have that instead.
• Baguette: Any crusty bread will do the job – try sourdough, ciabatta, or even thick slices of Italian bread. Just make sure it’s sturdy enough to hold up to the soup and cheese.
• Butter: Olive oil can replace butter if needed, though you’ll lose some of that rich, creamy flavor that butter brings to the caramelized onions.

Watch Out for These Mistakes While Cooking

The biggest mistake people make with slow cooker French onion soup is skipping the full cooking time for the onions – they need those 6 to 7 hours on HIGH to properly caramelize and develop that sweet, deep flavor that makes this soup special.

Resist the urge to stir the onions more than once during cooking, as opening the lid releases heat and adds time to the process, plus the onions caramelize better when left undisturbed.

When broiling, keep a close eye on your soup bowls since the cheese can go from perfectly melted to burnt in less than a minute – it’s ready when the cheese is bubbly and just starting to brown at the edges.

Finally, make sure your oven-safe dishes are truly broiler-safe (check the bottom for markings) and place them on a baking sheet before broiling to catch any drips and make handling easier.

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What to Serve With French Onion Soup?

French onion soup is pretty filling on its own with all that melted cheese and bread, but I love pairing it with a crisp green salad dressed with a simple vinaigrette to cut through the richness. A Caesar salad or arugula salad with lemon and olive oil works perfectly here. If you’re still hungry after, some roasted vegetables like Brussels sprouts or green beans make a nice side, or you could serve it alongside a charcuterie board with cured meats, olives, and pickles for a cozy dinner party. Since the soup is already pretty rich and cheesy, I’d stick with lighter sides that add some freshness to the meal.

Storage Instructions

Store: The soup base keeps really well in the fridge for up to 5 days in an airtight container. I actually think it tastes even better the next day after all those flavors have had time to hang out together. Just store it without the bread and cheese, and add those fresh when you’re ready to eat.

Freeze: French onion soup freezes great for up to 3 months. Let it cool completely, then portion it into freezer-safe containers, leaving a bit of room at the top for expansion. Again, skip adding the bread and cheese until you’re ready to serve.

Reheat: Warm the soup on the stovetop over medium heat until it’s nice and hot, stirring occasionally. You can also microwave individual portions, then top with your toasted baguette slices and cheese before broiling for that classic melty, bubbly top.

Estimated Nutrition

Estimated nutrition for the whole recipe (without optional ingredients):

• Calories: 2300-2500
• Protein: 80-90 g
• Fat: 90-100 g
• Carbohydrates: 320-340 g

Ingredients

For the soup base:

• 6 thyme sprigs
• 1.5 tsp sugar
• 1 bay leaf
• 5 cups beef broth (quality matters here for depth of flavor)
• 1 tbsp sherry vinegar
• 1/2 tsp black pepper (freshly ground preferred)
• 1.5 tsp kosher salt
• 1/4 cup unsalted butter (I use Kerrygold for richness)
• 2 lb Vidalia onions (sliced into 1/4-inch rings)

For the topping:

• black pepper
• fresh thyme leaves (optional garnish for color and aroma)
• 6 oz Gruyère cheese (freshly grated for even melting)
• 12 baguette slices (cut on slight diagonal, about 1/2-inch thick)

Step 1: Prepare Ingredients and Start Caramelizing Onions

• 2 lb Vidalia onions, sliced into 1/4-inch rings
• 1/4 cup unsalted butter
• 1.5 tsp sugar
• 1.5 tsp kosher salt
• 1/2 tsp black pepper, freshly ground

Slice the Vidalia onions into 1/4-inch rings—this size is crucial as it allows them to break down evenly and caramelize properly over the long cooking time.

Add the sliced onions, unsalted butter, sugar, kosher salt, and black pepper to your slow cooker.

The sugar won’t make the soup sweet; instead, it enhances the natural caramelization of the onions.

I’ve found that using quality butter like Kerrygold really adds a subtle richness to the final dish that budget butter can’t match.

Stir everything together to coat the onions evenly with butter.

Step 2: Add Aromatics and Begin Slow Cooking

• onion mixture from Step 1
• 6 thyme sprigs
• 1 bay leaf

Add the thyme sprigs and bay leaf to the slow cooker with the onion mixture.

These will infuse the onions as they cook and create a deep, complex base flavor.

Cover and cook on HIGH for 6 to 7 hours, stirring once halfway through (around the 3 to 3.5 hour mark) to ensure even caramelization.

The onions should become deeply golden and almost jam-like in consistency when fully cooked.

Step 3: Build the Soup Base with Broth and Vinegar

• caramelized onion mixture from Step 2
• 5 cups beef broth
• 1 tbsp sherry vinegar

Remove and discard the thyme sprigs and bay leaf from the caramelized onions.

Pour in the beef broth and sherry vinegar, stirring well to combine everything evenly.

The vinegar adds brightness and depth to the soup, cutting through the richness of the butter and caramelized onions.

Cook on HIGH for 30 minutes to allow the flavors to meld and the soup to heat through completely.

Step 4: Toast Bread and Prepare for Broiling

• 12 baguette slices, cut on slight diagonal, about 1/2-inch thick

While the soup finishes cooking, preheat your oven to broil and arrange the baguette slices on a baking sheet.

Toast them under the broiler for 1 to 2 minutes per side until golden and crispy—this prevents them from becoming soggy when topped with cheese and broth.

Set the toasted bread aside.

Pour the finished soup into 6 oven-safe dishes (such as traditional French onion bowls or small ramekins), leaving about 1/2 inch of space at the top.

Step 5: Assemble and Broil with Cheese Topping

• soup from Step 3
• toasted baguette slices from Step 4
• 6 oz Gruyère cheese, freshly grated

Top each bowl of soup with 2 toasted baguette slices, overlapping them slightly if needed.

Divide the freshly grated Gruyère cheese evenly among the bowls, using about 1 ounce (roughly 1/4 cup) per bowl and piling it generously over the bread.

The fresher grated cheese melts much more evenly than pre-shredded varieties.

Broil for about 3 minutes, watching carefully, until the cheese is melted and bubbly with light golden-brown spots on top.

Step 6: Garnish and Serve

• fresh thyme leaves, optional garnish
• black pepper, for finishing

Remove the bowls from the broiler and let them cool for 1 to 2 minutes—the cheese will be extremely hot.

Garnish each bowl with a few fresh thyme leaves for color and aroma, and finish with a crack of freshly ground black pepper.

Serve immediately while the soup is still steaming and the cheese is at its creamiest.

Simple Slow Cooker French Onion Soup

Delicious Simple Slow Cooker French Onion Soup recipe with step-by-step instructions.

Print Recipe Pin Recipe

Prep Time 2 hours hrs 30 minutes mins

Cook Time 5 hours hrs

Total Time 7 hours hrs 30 minutes mins

Servings 6 servings

Calories 2400 kcal

Ingredients

  

For the soup base:

• 6 thyme sprigs
• 1.5 tsp sugar
• 1 bay leaf
• 5 cups beef broth (quality matters here for depth of flavor)
• 1 tbsp sherry vinegar
• 1/2 tsp black pepper (freshly ground preferred)
• 1.5 tsp kosher salt
• 1/4 cup unsalted butter (I use Kerrygold for richness)
• 2 lb Vidalia onions (sliced into 1/4-inch rings)

For the topping:

• black pepper
• fresh thyme leaves (optional garnish for color and aroma)
• 6 oz Gruyère cheese (freshly grated for even melting)
• 12 baguette slices (cut on slight diagonal, about 1/2-inch thick)

Instructions

 

• Slice the Vidalia onions into 1/4-inch rings—this size is crucial as it allows them to break down evenly and caramelize properly over the long cooking time. Add the sliced onions, unsalted butter, sugar, kosher salt, and black pepper to your slow cooker. The sugar won't make the soup sweet; instead, it enhances the natural caramelization of the onions. I've found that using quality butter like Kerrygold really adds a subtle richness to the final dish that budget butter can't match. Stir everything together to coat the onions evenly with butter.
• Add the thyme sprigs and bay leaf to the slow cooker with the onion mixture. These will infuse the onions as they cook and create a deep, complex base flavor. Cover and cook on HIGH for 6 to 7 hours, stirring once halfway through (around the 3 to 3.5 hour mark) to ensure even caramelization. The onions should become deeply golden and almost jam-like in consistency when fully cooked.
• Remove and discard the thyme sprigs and bay leaf from the caramelized onions. Pour in the beef broth and sherry vinegar, stirring well to combine everything evenly. The vinegar adds brightness and depth to the soup, cutting through the richness of the butter and caramelized onions. Cook on HIGH for 30 minutes to allow the flavors to meld and the soup to heat through completely.
• While the soup finishes cooking, preheat your oven to broil and arrange the baguette slices on a baking sheet. Toast them under the broiler for 1 to 2 minutes per side until golden and crispy—this prevents them from becoming soggy when topped with cheese and broth. Set the toasted bread aside. Pour the finished soup into 6 oven-safe dishes (such as traditional French onion bowls or small ramekins), leaving about 1/2 inch of space at the top.
• Top each bowl of soup with 2 toasted baguette slices, overlapping them slightly if needed. Divide the freshly grated Gruyère cheese evenly among the bowls, using about 1 ounce (roughly 1/4 cup) per bowl and piling it generously over the bread. The fresher grated cheese melts much more evenly than pre-shredded varieties. Broil for about 3 minutes, watching carefully, until the cheese is melted and bubbly with light golden-brown spots on top.
• Remove the bowls from the broiler and let them cool for 1 to 2 minutes—the cheese will be extremely hot. Garnish each bowl with a few fresh thyme leaves for color and aroma, and finish with a crack of freshly ground black pepper. Serve immediately while the soup is still steaming and the cheese is at its creamiest.