Melt butter in a large saucepan and cook the onions on a low heat, covered for 10-15 minutes, stir from time to time to make sure don't overbrown. Remove the lid and cook onions until they are browned, add the stock, bouquet garni (or herbs) and seasoning. Cover and simmer for 30 minutes, then remove the bouquet garni, if using. Continue to cook for 20 minutes. Quickly add the cheese and stir, serve straight away. Serve with French bread.