Begin by thinly slicing 4 large yellow onions and setting them aside.

In a large pot or Dutch oven, melt 3 tablespoons of unsalted butter along with 1 tablespoon of olive oil over medium heat.

Sprinkle 1 teaspoon of salt and 1 teaspoon of sugar over the onions and continue cooking on medium-low heat for about 40 minutes.

Pour in 4 cups of beef broth and 1 cup of dry white wine. Add 1 tablespoon of fresh thyme and 1 bay leaf.

Once boiling, reduce the heat to low and let the soup simmer gently for 30 minutes.

While your soup is simmering, slice a baguette into 1-inch thick pieces and toast them.

Discard the bay leaf and ladle the soup into oven-safe bowls, topping with baguette slices and 2 cups of grated Gruyère cheese.

Broil the bowls in the oven for about 2-3 minutes until the cheese is bubbly and golden.

Carefully remove the bowls from the oven and serve warm.