Baked Zucchini and Yellow Squash Casserole

There’s something comforting about a good casserole, isn’t it? I’ve always loved how they bring together simple ingredients to make something that feels like a warm hug. This zucchini and yellow squash casserole has become a regular at our family dinners, especially during summer when the garden is overflowing with squash.

I love making this dish because it’s so straightforward – no fancy techniques or hard-to-find ingredients needed. Just honest, good food that comes together without much fuss. Plus, it’s a great way to use up those extra vegetables that seem to multiply in the crisper drawer. I usually prep everything while I’m already in the kitchen making dinner, which means less cleanup later.

Whether you’re looking for a side dish for your Sunday dinner or trying to sneak more vegetables onto your family’s plates, this casserole might just become your new go-to recipe. It’s certainly become mine!

What Kind of Squash Should I Use?

This recipe calls for both zucchini and yellow squash, which are perfect summer squash partners in the kitchen. When shopping, look for medium-sized squash that’s about 6-8 inches long – the really big ones tend to be more watery and have larger seeds that can make your casserole soggy. Fresh summer squash should feel firm and heavy for its size, with smooth, blemish-free skin. While you could use all zucchini or all yellow squash in this recipe, using both gives you a prettier color contrast and slightly different textures – yellow squash is a bit sweeter and softer, while zucchini has a mild flavor and holds its shape better when cooked.

Options for Substitutions

This casserole recipe is pretty adaptable – here are some helpful swaps if you need them:

• Zucchini and yellow squash: You can use all zucchini or all yellow squash if you can’t find both. Patty pan squash works great too. Just keep the total amount of squash the same.
• Fontina cheese: No fontina? Try using Gruyere, provolone, or even mozzarella. Each will give a slightly different flavor, but they all melt nicely and work well in this dish.
• Panko breadcrumbs: Regular breadcrumbs work fine here, or try crushed butter crackers for a different twist. If you’re going gluten-free, crushed pork rinds or gluten-free breadcrumbs make good alternatives.
• Full-fat milk: You can use 2% milk, but the sauce won’t be as creamy. For extra richness, try using half-and-half or a mix of milk and cream.
• Parmesan cheese: Pecorino Romano or Grana Padano make good swaps for Parmesan. They’re both hard Italian cheeses with similar salty flavors.
• Plain flour: For a gluten-free version, use cornstarch (2 tablespoons) or your favorite cup-for-cup gluten-free flour blend.

Watch Out for These Mistakes While Cooking

The biggest challenge when making squash casserole is dealing with excess moisture – to avoid a watery dish, make sure to slice your zucchini and yellow squash into even 1/4-inch rounds and salt them for at least 20 minutes before cooking, then pat them thoroughly dry with paper towels. Another common mistake is undercooking the flour-butter mixture (roux) for the sauce – cook it for at least 1-2 minutes while stirring constantly to get rid of the raw flour taste and achieve a silky-smooth sauce. To prevent a soggy topping, avoid pressing the breadcrumb mixture too firmly onto the casserole, and wait until the last 15 minutes of baking to add it – this ensures you’ll get that perfect golden-brown, crispy crust. For the best texture, let the casserole rest for 10-15 minutes after baking, which allows the sauce to thicken and makes it easier to serve clean portions.

What to Serve With Squash Casserole?

This cheesy squash casserole works perfectly as a side dish, but you can also make it the star of your meal with some tasty accompaniments. A simple grilled chicken breast or roasted pork tenderloin would pair really well with the creamy, cheesy flavors of the casserole. For a vegetarian meal, serve it alongside a fresh green salad with cherry tomatoes and a light vinaigrette dressing – the crisp, cool salad provides a nice contrast to the warm, rich casserole. You could also add some crusty French bread or dinner rolls to round out the meal and soak up any extra cheese sauce.

Storage Instructions

Keep Fresh: This squash casserole stays good in the fridge for up to 4 days when stored in an airtight container. The cheese sauce might thicken up a bit in the fridge, but don’t worry – that’s totally normal! It’s perfect for making ahead for family dinners or meal prep.

Freeze: While you can freeze this casserole, the texture of the squash might change a bit when thawed. If you want to freeze it, wrap it tightly in foil and pop it in a freezer bag – it’ll keep for up to 2 months. Just remember to thaw it in the fridge overnight before reheating.

Reheat: To warm up your casserole, cover it with foil and pop it in the oven at 350°F for about 20-25 minutes until heated through. If you’re in a hurry, you can microwave individual portions, but the breadcrumb topping won’t be as crispy. A quick tip: sprinkle a little extra cheese on top when reheating to bring back that gooey goodness!

Preparation Time	30-45 minutes
Cooking Time	40-50 minutes
Total Time	70-95 minutes
Level of Difficulty	Medium

Estimated Nutrition

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

• Calories: 2000-2200
• Protein: 50-60 g
• Fat: 130-150 g
• Carbohydrates: 150-170 g

Ingredients

For the casserole:

• 1/4 tsp ground mustard
• 8 oz fontina cheese (shredded, I use Stella Fontina)
• 1/2 tbsp unsalted butter
• 1.5 lb zucchini
• 1/4 tsp ground black pepper
• 1.5 lb yellow squash
• 1.5 tsp kosher salt
• 1/4 cup all-purpose flour
• 3 cups whole milk (room temperature for a smoother sauce)
• 1/2 tsp paprika
• 4 garlic cloves (minced for best flavor)

For the béchamel sauce:

• 4 tbsp unsalted butter (I like Kerrygold for this)

For the topping:

• 1/4 cup grated Parmesan cheese
• 2 tsp olive oil
• 1/2 cup panko breadcrumbs (for extra crispiness)

Step 1: Prepare the Squash and Béchamel Sauce

Start by placing 1/2 tablespoon of unsalted butter in an 8×11-inch or other 2-quart baking dish and set it aside to let the butter come to room temperature.

Using a mandoline or chef’s knife, trim and very thinly slice 1 1/2 pounds of zucchini and 1 1/2 pounds of yellow summer squash crosswise.

Place the sliced vegetables in a colander, sprinkle with 1 1/2 teaspoons of kosher salt, and toss to combine.

Place the colander in the sink or over a large bowl and let the squash drain for 20 minutes.

Step 2: Preheat Oven and Make Béchamel Sauce

Meanwhile, arrange a rack in the middle of the oven and heat the oven to 350ºF.

Finely chop 4 garlic cloves.

Melt the remaining 4 tablespoons of unsalted butter in a medium saucepan over medium heat.

Add the garlic and cook, stirring occasionally, until fragrant, about 1 minute.

Add 1/4 cup of all-purpose flour and cook, whisking, until the garlic and flour are lightly toasted, about 2 minutes.

Step 3: Finish the Béchamel Sauce

While whisking constantly, slowly pour in 3 cups of milk.

Increase the heat to medium-high and bring to a boil, stirring constantly.

Reduce the heat to maintain a simmer and cook, whisking often, until the sauce thickens significantly, roughly 5 to 10 minutes.

Remove from the heat.

Add 1/2 teaspoon paprika, 1/4 teaspoon dry mustard, the remaining 1/4 teaspoon kosher salt, and 1/4 teaspoon black pepper, and whisk to combine.

Step 4: Assemble the Squash Gratin

When the squash is ready, work with a heaping handful at a time, place in a clean kitchen towel or paper towels and squeeze as much water as possible from the squash, then place in a bowl.

Shred 8 ounces of fontina cheese on the large holes of a box grater.

Coat the baking dish with the softened butter.

Spread 1/2 cup of the béchamel sauce in the bottom of the baking dish.

Spread half of the squash over the cream sauce in an even layer.

Drizzle with half of the remaining béchamel sauce (about 1 cup), then sprinkle with half of the fontina cheese.

Layer on the remaining squash, béchamel sauce, and fontina cheese.

Step 5: Add Topping and Bake

Place 1/2 cup of panko breadcrumbs, 1/4 cup of grated Parmesan cheese, and 2 teaspoons of olive oil in a small bowl.

Season with a pinch of kosher salt and black pepper, and stir to combine.

Sprinkle the breadcrumb mixture evenly over the casserole.

Bake until the sides of the casserole are bubbling, about 40 minutes.

Check after 30 minutes and tent the casserole loosely with aluminum foil if the top is already browned before continuing baking.

Let rest for 15 minutes before serving.