Slice the onions thinly. Then melt the butter in a Dutch oven and add the onions and salt. Cover the pot with the lid and heat the onions on medium low heat for 5 minutes. Then take the top off and let the onions caramelize, for at least 20 minutes on low heat. Stir occasionally.
2 large yellow onions, 3 tbsp unsalted butter, ½ tsp salt
Heat the broth. While the onions caramelize, add the beef broth into a saucepan over low heat until it's warmed throughout. In the meantime, grate the cheese and slice your bread.
1 quart beef broth
After the onions are a dark golden brown, add the wine to them and let the mixture come to a boil for about 2 minutes. Then add the flour and stir until incorporated and the mixture is thickened.
½ cup dry white wine, 1 tbsp all purpose flour
Add the broth to the onions and let it all boil for about 10 minutes.
Spoon the soup into your oven-safe bowls and top each one with a piece of bread (you might have to cut the bread slice into a few pieces). Cover each soup bowl with enough cheese that the surface is covered - grate more if you need it. Place the bowls on a baking sheet and place them in the oven. Turn the broiler on for about 5 minutes, but watch closely so that the cheese doesn't burn. Remove once the cheese is golden brown and enjoy.
4 slices fresh bread, ½ cup gruyere cheese