Fry the finely diced onions with minced meat in 20 g butter. Add tomato puree and season with salt, pepper and mustard.
Melt the remaining butter, sweat the flour and deglaze with milk. Add the parmesan and processed cheese and let melt while stirring constantly. Season well with salt, pepper and nutmeg.
Boil and quench pasta.
Put half of the pasta in a greased baking dish. Pour the minced meat and tomato sauce over it. Then spread the rest of the pasta and the cheese sauce on top. Cover the casserole with the Gouda slices and bake in the preheated oven on the middle rack at 200 ° C for about 40 - 50 minutes.