Begin by thinly slicing the onions.

In a large pot, melt the butter and olive oil over medium heat.

Add the onions and sprinkle with salt and sugar.

Cook the onions for about 30-40 minutes, stirring occasionally, until they are deep golden brown.

Pour in the white wine and scrape the bottom of the pot.

Then, add the beef broth, thyme, and bay leaf.

Bring the mixture to a gentle boil, then reduce the heat and let it simmer for 20-30 minutes.

While the soup simmers, toast the slices of French bread until golden.

Remove the bay leaf from the soup.

Ladle the soup into oven-safe bowls and place a slice of toasted bread on top.

Sprinkle the grated Gruyère cheese generously over the bread.

Place under the broiler until the cheese is bubbly and golden brown.