Thinly slice 5 to 6 sweet yellow onions and set them aside.

In a large pot, melt 3 tablespoons of unsalted butter and 1 tablespoon of olive oil over medium heat.

Add the sliced onions to the pot and stir gently to coat them with the butter mixture.

Sprinkle a pinch of salt and 1 teaspoon of sugar over the onions and cook on medium-low heat for 40 minutes, stirring occasionally.

Once caramelized, pour in 6 cups of beef broth or vegetable broth and 1/2 cup of dry white wine.

Add a few sprigs of fresh thyme and a bay leaf, then bring the mixture to a gentle boil.

Reduce heat to low and let the soup simmer uncovered for 30 minutes, tasting and seasoning with black pepper as needed.

Slice a stale baguette into thick pieces and toast them in a preheated oven at 400°F for about 10 minutes.

Ladle hot soup into oven-safe bowls, place a toasted baguette slice on top, and sprinkle with shredded Gruyère cheese.

Broil the bowls in the oven for about 2-3 minutes until the cheese melts and bubbles.

Once golden, carefully remove from oven and let cool slightly before serving.