Cut two large onions in thin slices and fry them in butter till very brown. Add them to about a pound of potato purée, add a little more salt and pepper, and put the mixture, well worked, in a buttered fireproof dish. The dish should be rather flat and the mixture not more than three-quarters of an inch thick. Add a few pieces of butter on the top and brown well in the oven.
You can also do this dish in a frying pan, frying first one side, then the other. It gives it a slightly different taste.