Start by heating the butter and olive oil in a large pot over medium heat. Add the sliced onions and stir to coat them evenly. Sprinkle with a pinch of salt and, if using, the sugar.

Cook, stirring occasionally, for 25–30 minutes, or until the onions are deeply golden and caramelized. Be patient this step is key to developing the soup’s signature flavor.

Stir in the minced garlic and cook for about 1 minute, just until fragrant. Pour in the ½ cup beef broth + 1 teaspoon apple cider vinegar to deglaze the pot, scraping up any browned bits from the bottom. Let it simmer for 2–3 minutes to reduce slightly.

Add the beef broth, thyme, and bay leaf to the pot. Bring the soup to a gentle simmer, then reduce the heat to low. Let it cook for 20–25 minutes, allowing the flavors to meld together. Season with salt and pepper to taste, and remove the bay leaf before serving.

While the soup simmers, preheat your broiler. Toast the baguette slices until golden brown. Ladle the soup into oven-safe bowls, place a slice of baguette on top of each, and sprinkle generously with grated Gruyère cheese.

Place the bowls under the broiler for 2–3 minutes, or until the cheese is melted, bubbly, and slightly browned. Keep an eye on them to avoid burning. Carefully remove the bowls (they’ll be hot!) and let them cool slightly before serving.