For the goose gratin, peel onion and cut into small pieces. Steam cook potato, peel and cut into pieces. Cut dumplings into pieces.
Chop goose meat (leftovers) into bite-sized pieces. Heat goose fat in a deep pan, sauté onion in it.
Add potato, dumplings and goose meat, season with salt and pepper and fry until crispy. Turn often.
Arrange the goose gröstl and serve.