Rub the mushrooms dry, cut into slices. Peel and dice the onion. Mix 4 tablespoon flour, pepper and salt in a deep plate. Wash the chicken fillets and pat dry. Cook the pasta in salted water according to the instructions on the packet.
Turn the chicken breast fillets in the seasoned flour. Heat half of the butter in a large pan. Fry the chicken breast fillets over medium heat for about 8 minutes on both sides. Add the onion and mushrooms to the pan with the poultry and cook for 5 minutes. Cover and set aside.
Heat the rest of the butter in a saucepan and sweat the rest of the flour in it. Gradually pour in the broth while stirring, simmer for about 5 minutes. Mix in the sour cream, season with mustard, salt and pepper.
Mix the sauce with the poultry, mushrooms and onions. Serve with the pasta. The sauce gets a special taste if you add a little dry white wine.