Peel and finely chop the onion. Grate cheese separately. Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: level 2). Cook pasta in 3-4 l boiling salted water for 6-7 minutes.
Melt 3 tablespoons of butter in a saucepan. Fry the onion until transparent. Dust with flour and sweat lightly for 1-2 minutes, stirring continuously - the roux should not turn brown.
Pour in the milk, stirring vigorously all the time so that no lumps form. Bring to the boil and simmer for about 5 minutes. Melt the cheese, except for 2 tbsp. parmesan, in the sauce while stirring.
Season the sauce with salt, pepper and mustard. Drain the pasta and pour it back into the pot. Mix immediately with the sauce so that the noodles do not stick together.
Divide into 4 small or 1 large casserole dish. Crumble the cornflakes over it. Sprinkle with the rest of the Parmesan and spread 1 tablespoon of butter in flakes on top. Bake in the oven for about 20 minutes.
Wash the meat, dab dry and cut into 8 medallions. Fry in hot oil for 3-4 minutes on each side. Season with salt and pepper. Serve with the macaroni.