In a large pot, melt 4 tablespoons of butter and 2 tablespoons of olive oil over medium heat. Once melted, add about 3 large, sliced onions, along with a pinch of salt and pepper. Stir to coat the onions in the fat. Cook uncovered for 45–55 minutes, stirring occasionally until they are deeply caramelized and golden brown; lower the heat if they start to brown too quickly.

Pour in 1/2 cup of white wine and 1/4 cup of dry sherry into the pot with the caramelized onions. Increase the heat to medium-high and bring the mixture to a gentle boil. Allow the liquid to reduce for about 5–6 minutes until it thickens slightly and the alcohol cooks off.

Sprinkle 2 tablespoons of flour over the onions and stir continuously for approximately 1 minute, allowing the flour to cook and absorb the flavors.

Gradually pour in 6 cups of beef broth, stirring to combine thoroughly. Toss in a bay leaf, 1 teaspoon of thyme, and 1 tablespoon of Worcestershire sauce. Bring the soup to a simmer and let it cook for about 20 minutes.

While the soup is simmering, preheat your oven to 400°F (200°C). Slice a baguette into 1-inch thick pieces and place them on a baking sheet. Bake the slices for 8–10 minutes until golden and crispy.

Ladle the hot French Onion Soup into ovenproof bowls and top each bowl with the toasted baguette slices. Generously sprinkle with grated Gruyère cheese.

Place the bowls under the broiler for about 2–4 minutes, watching closely as the cheese melts and turns golden brown.