Melt 2 tablespoons of butter and 1 tablespoon of olive oil over medium heat in a large skillet. Add 3 large, thinly sliced yellow onions, 2 sprigs of fresh thyme, and 1 bay leaf. Cook on low heat, stirring every 10 minutes until the onions are deeply caramelized, about 40-45 minutes. Add a pinch of salt to enhance sweetness.

Once the onions are caramelized, pour in 1/2 cup of white wine to deglaze the skillet, scraping up any browned bits. Let reduce for a couple minutes until the alcohol evaporates. Add 4 cups of beef broth and bring to a gentle boil. Simmer while preheating the broiler.

Ladle the hot soup into oven-safe bowls. Place a few toasted baguette slices on top of each bowl and sprinkle with 1 cup of grated Gruyère cheese.

Transfer the bowls to a preheated broiler on a baking sheet. Broil for 3-5 minutes until the cheese is melted and golden brown.

Carefully remove the bowls, using oven mitts. Garnish with fresh thyme if desired and serve immediately.