Place onions and butter in saucepan and cook slowly until gojden brown.
Add flour, mix and cook a few minutes.
Add the stock.
Pour into earthenware casserole, sprinkle on it half the cheese, float toasted slices of bread on top, and sprinkle with remainder of the cheese.
Place in hot oven until cheese has melted and browned.
It is important to use freshly grated cheese as it melts on the soup in great globs instead of clouding the soup like powdered cheese.
(Try Gruyere cheese.)