For the spaetzle, wash the spinach, dab dry well and chop. Finely mash the eggs and spinach. Mix 300 g flour, 1 tsp salt and nutmeg. Add egg and spinach mixture and approx. 5 tbsp. mineral water. Beat everything with a stirring spoon with a hole until a shiny dough is formed which throws bubbles. Let it rest for about 30 minutes.
For the fillets, wash and clean the mushrooms, dab dry and cut in half. Peel and finely chop the onion. Wash the fillets and dab dry. Brush with mustard, season with pepper and wrap each with 5 slices of bacon. Heat oil in a pan. Fry the fillets for about 5 minutes. Place in an ovenproof dish and roast in a preheated oven (electric cooker: 150 °C/circulating air: 125 °C/gas: see manufacturer) for approx. 20 minutes. Set the pan aside.
For the spaetzle, boil up plenty of salted water in a wide pot. Fill the spaetzle dough in portions into a spaetzle press and press it into the boiling water. Simmer the spaetzle for 1-2 minutes until they float on the surface. Lift them out with a skimmer, put them into a sieve, rinse with cold water and drain well.
For the sauce, heat the pan with frying fat. Sauté the mushrooms in it in portions for about 5 minutes. Add onion and fry briefly. Dust everything with 1 tbsp. flour and sauté briefly. Deglaze with cognac, 1⁄4 l water and cream. Bring to the boil, stir in broth and simmer for about 5 minutes. Season to taste with salt and pepper.
Remove the fillets and let them rest for about 5 minutes. Stir the gravy into the sauce. Heat butter in a pan. Swivel the spaetzle in it. Arrange everything.