Fry the goose breast in melted clarified butter on all sides, deglaze with a little broth and let simmer covered for about 30 minutes (add some broth if necessary).
Clean the Brussels sprouts and add to the goose breast with the diced potatoes and the remaining stock. Season the stew with salt, pepper and nutmeg and stew for about 25 minutes. Remove the goose breast and cut into slices. Sauté the onion cubes in butter and stir into the Brussels sprouts with the sour cream. Season to taste and place the meat slices on top.