Cook the spaetzle according to the instructions on the packet, but without salt. Drain the mushrooms and collect the liquid. Make up to 200 ml with water.
Fry the Leberkäse together with the onions. Add the mushroom water, bring to the boil and stir in the cream sauce. Add the mushrooms (I always take the small whole ones and cut them in half), put everything in a baking dish, sprinkle with the cheese (if you want it to go quickly, I mix in the cheese) and bake in the oven at 180 ° C until the cheese has melted nicely (approx. 17 min.).