Peel and quarter the potatoes and cook in salted water.
Peel the onions, cut in half and then cut into fine strips. Peel, core, quarter and thinly slice the apples.
Heat the oil in a pan and first fry the onion strips over a medium heat until they have slightly taken on color and lost volume. Then add the apple slices and cook until the apple slices are soft and have also taken on a light color. Season the apple and onion vegetables with salt and marjoram and set aside.
Fry the sausages in another pan.
Drain the finished, boiled potatoes and process with milk and butter to a fluffy mashed potatoes. Arrange everything on plates and serve hot.