Melt the butter in a casserole and simmer the onion rings over a low heat in about 20 minutes until they are nice and translucent until they are almost brown.
Cut the bread or rolls into slices and line an ovenproof bowl or casserole dish with them. Spread the onions on top and sprinkle the cheese on top. Pour on the hot broth and grind a good amount of black pepper over it. Cook in a preheated oven at 180 ° C for about 20 minutes.