Mix flour, eggs, 1 teaspoon salt, 1 pinch of freshly grated nutmeg and mineral water with the dough hooks of the mixer to a thick dough. Then beat vigorously with a wooden spoon (with hole) until the dough starts to bubble.
The dough should flow viscously from the spoon without tearing. Cover and leave to rest for about 1 hour.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Bring plenty of salted water to the boil in a large pot. For hand-scraped spaetzle, moisten a spaetzle board with bevelled edge and a long knife in the cooking water.
Gradually spread about 2 tbsp. of dough thinly on the board and scrape fine strips with the back of a knife directly into the boiling water.
For traditional hand-scraped spaetzle some practice is necessary. It is easier with the press: Add dough in portions and press directly into boiling water, cutting the spaetzle with a knife for the right length.
Prepare the dough with less water so that it is not so sticky. The spaetzle are cooked when they rise to the surface. Lift them out with a skimmer, put them in a sieve and rinse with cold water.
Drain well.
Put the spaetzle in four small or one large ovenproof greased dish. Spread the roasted onions on top and sprinkle with cheese. Bake in a hot oven for 10-15 minutes.
In the meantime heat 2 tablespoons of oil in a large frying pan. Brown the meat in it in portions all around for about 4 minutes. Season with salt and pepper and remove. Heat 2 tbsp. oil in the frying fat. Fry the mushrooms for about 5 minutes, turning them over.
Season with salt and pepper. Dust with 2 tablespoons of flour and sauté briefly. Deglaze with wine, cream and 150 ml water, bring to the boil and simmer for about 5 minutes. Heat meat in the sauce again. Season to taste with salt and pepper.
Arrange spaetzle and fillet in mushroom cream.