Cook the pasta in plenty of boiling salted water for about 10 minutes. Steam deep-frozen vegetables in little salted water for about 5 minutes. Peel and finely dice onions. Drain the vegetables.
Drain the pasta and rinse with cold water. Heat the fat in a saucepan. Fry the onion cubes until translucent. Stir in flour and sweat it. Deglaze with stock and milk. Boil up while stirring.
Season to taste with salt, pepper and nutmeg. Peel the skin off the sausage. Cut the sausage into cubes. Grate cheese roughly. Grease an oven dish. Mix noodles, sausage and vegetables. Put pasta mixture, sauce and grated cheese alternately into the casserole dish.
Bake in the preheated oven (electric oven: 175 °C/ gas: level 2) for approx. 30 minutes. A cool beer goes well with it.