Mix 300 g flour, eggs, 1 teaspoon salt and mineral water to a thick dough. Let dough rest for about 1 hour. Peel onions and cut into rings. Season onion rings with salt, dust with 2 tbsp. flour. Heat 100 ml oil and fry the onion rings in it in portions for about 3 minutes each.
Bring salted water to a boil in a wide pot. Put the dough in portions into a spaetzle press. Press into the water and cook for about 3 minutes until the spaetzle rise to the top. Take them out with a skimmer, quench them briefly with cold water and let them drip off
Grate the cheese roughly. Season the spaetzle with salt and nutmeg and fill into ovenproof dishes. Spread onions on top and sprinkle with cheese. Bake in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: see manufacturer) for 10-15 minutes
Meanwhile clean and trim the mushrooms and, depending on their size, halve or quarter them. Cut the fillets lengthwise into quarters and dice them. Heat 2 tablespoons of oil in a large frying pan and fry the meat for approx. 4 minutes, turning it well. Season with salt and pepper. Remove from the pan, put 2 tbsp. oil in the pan and fry the mushrooms for approx. 5 minutes while turning. Season with salt and pepper, deglaze cream. Add meat and warm up in the sauce. Arrange spaetzle and sliced meat
Waiting time approx. 50 minutes