Melt butter and oil in your pot over medium heat.
Add sliced onions and stir them well.
Sprinkle sugar over the onions to help caramelization.
Cook slowly for 40 minutes, stirring occasionally.
The onions should turn deep golden brown.
Pour in the white wine to deglaze the pot.
Scrape up any brown bits from the bottom.
Add beef broth and bay leaves next.
Bring everything to a gentle boil.
Reduce heat and let it simmer for 30 minutes.
Season with salt and pepper to your taste.
Remove the bay leaves before serving.
Toast French bread slices until lightly golden.
Place them on top of each soup bowl.
Sprinkle generous amounts of Gruyère cheese over bread.
Put bowls under the broiler for 2-3 minutes.
Watch carefully so cheese doesn't burn.
The cheese should be melted and bubbly.