Cook the pasta in boiling salted water according to the instructions on the packet until just firm to the bite. Peel and finely chop the onion. Melt 3 tablespoons of butter in a saucepan over medium heat. Fry the onion until transparent.
Dust with flour and sweat. Add milk little by little while stirring constantly. Season with pepper, salt and mustard. Simmer at low heat for about 5 minutes. Drain the pasta and let it drain well.
Stir the cheese, except for 25 g Parmesan, into the béchamel sauce and let it melt. Mix the pasta with the sauce and fill into small casseroles. Crumble the cornflakes slightly and sprinkle them over the pasta with 25 g Parmesan cheese.
Place 20 g butter in flakes on top. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) for 15-20 minutes. Wash meat, pat dry and cut into 10-12 pieces.
Heat the oil in a large frying pan. Fry the turkey steaks in it for about 2 minutes on each side. Arrange the pasta-cheese casserole with the turkey steaks on plates.