Napolitan

Boil the spaghetti in a large pot of salted water until al dente. Reserve about 100 ml of the pasta cooking water.

Slice the sausages into rounds about 1 cm thick. Slice the onion into thin strips and the green pepper into long strips. If using button mushrooms, slice them.

Heat vegetable oil in a large skillet. Sauté the sausages for 2–3 minutes until lightly browned.

Add the onion, green pepper, and mushrooms. Cook for another 4–5 minutes until the onion is tender.

Add the ketchup and tomato paste directly to the skillet. Cook for 1–2 minutes, stirring constantly—this intensifies the flavor and mellows out the sharpness of the ketchup.

Pour in 50–100 ml of the reserved pasta water, and season with salt and black pepper. Stir until a smooth sauce forms.

Transfer the cooked spaghetti into the skillet and toss thoroughly so every strand is coated in the sauce.

Add the butter and cook for another 1–2 minutes until completely melted.