Peel the onions and chop roughly. Melt the butter in a pan and sauté the onions.
Once they are brown, sprinkle in flour and stir well.
Pour white wine and broth over them and allow to boil.
In a large ovenproof bowl, place one slice of bread, then one slice of cheese and so on.
Then pour the contents of the pan over the bread and cheese and cook in oven for approximately 30 minutes at 325°.
Ladle into four individual bowls and serve immediately.