Peel the onion and cut into small cubes, also cut the meat sausage into small cubes.
Fry both in the pot. Then add the tomatoes and, depending on your taste, milk and bring everything to a boil. Now add the roux and let the sauce cook for another minute. Tie up as you like, bringing the sauce to the boil again. Finally, refine everything to taste with ketchup and season with salt and pepper.
Cook the pasta according to the instructions and serve with the sauce.