Clean, wash and possibly halve the Brussels sprouts. Cook in 1/2 litre boiling salted water for about 20 minutes. Cook the pasta in plenty of boiling salted water for about 10 minutes. Cut the sausage into slices, halve the slices.
Drain the pasta and rinse with cold water. Drain the Brussels sprouts and collect the vegetable water. Grate cheese. Peel onions, dice finely and fry in hot fat. Dust with flour and sauté.
Add milk and Brussels sprout water while stirring. Simmer for 5 minutes at low heat. Stir in 125 g cheese. Season sauce with salt and pepper. Put the noodles, Brussels sprouts and sausage in a large casserole dish and pour the béchamel sauce over it.
Sprinkle with remaining cheese and bake in the preheated oven (electric oven: 200 °C/ gas: level 3) for approx. 35 minutes. Wash parsley, dab dry, chop and sprinkle over the casserole.