Onions peel and chop finely. Clean the mushrooms, wash them briefly and cut into slices. Wash the fillet, dab dry and cut into strips. Heat 2 tablespoons of oil in a large deep frying pan.
Fry the fillet in portions while turning. Season with salt and pepper. Remove.
Heat 2 tablespoons of oil in hot frying fat. Sauté onions in it until transparent. Steam mushrooms for about 5 minutes. Season with salt and pepper. Dust with flour and sauté briefly. Add 200 ml water and milk while stirring.
Bring to the boil and simmer for about 3 minutes. Dice blue cheese and melt in the sauce. Stir in meat.
Meanwhile cook the noodles in boiling salted water according to the instructions on the packet. Wash and chop the parsley. Drain noodles, rinse and drain well. Mix with fillet-mushroom sauce and parsley.
Season to taste with salt and pepper. Put everything into a large casserole dish. Grate the cheese and sprinkle over it. Bake in the preheated oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer) for approx. 20 minutes.