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.