Cook the spaetzle in salted water according to the instructions on the packet.
In the meantime, cut the pork fillet into approx. 1 cm wide slices. Press wide with the knife. Heat the fat in a pan and briefly fry the fillet slices on both sides.
Place the meat in an ovenproof baking dish and cover with thin herb butter slices.
Then roast the finely chopped onion in the pan. Add the drained mushrooms and sauté briefly. Pour the meat stock and thicken the sauce with crème fraîche. Mix the cooked spaetzle with the mushroom sauce and mix with half of the grated cheese.
Spread the mixture on the fillet slices, sprinkle with the other half of the cheese and bake in the preheated oven for approx. 25 - 30 minutes at 220 ° C.