Begin by peeling and thinly slicing four large yellow onions. Heat a large, heavy-bottomed pot over medium heat and add 4 tablespoons of butter until melted.

Add the sliced onions to the pot, stirring to coat them evenly in the melted butter. Cook for about 30-40 minutes until deep golden brown.

Pour in 1/2 cup of white wine, using a wooden spoon to scrape up flavorful bits. Let it simmer for 5 minutes.

Pour in 6 cups of beef broth, stir in 1 teaspoon of thyme and a bay leaf. Bring to a gentle boil, then reduce heat and let simmer for 20-30 minutes.

Preheat the oven to 350°F (175°C). Slice French bread thickly and arrange on a baking sheet. Top each slice with Gruyère and Parmesan cheese and toast for about 10 minutes.

Discard the bay leaf and ladle the soup into oven-safe bowls. Place the cheese-topped bread on top and return to the oven for another 5-10 minutes until the cheese is melted.