Boil the spaetzle in salted water for about 8-10 minutes or until they float on top. Remove and drain.
Brush the mushrooms, cut off the stem and quarter the mushrooms. Peel the onions and cut into fine rings. Cut the cheese into fine strips.
Fry the sausages in 2 tablespoons of heated oil, remove and cut into pieces. Put the mushrooms and onions with 2 tablespoons of oil in the remaining frying fat, golden brown
fry and season with salt and pepper.
Layer the spaetzle, sausage pieces, mushroom-onion mixture and cheese alternately in a baking dish, cover with cheese and cook in the oven for about 15-20 minutes.
Mix the sour cream with the horseradish and season with salt and pepper.