Boil the spaghetti noodles.
Thinly slice the onion, bell pepper, and sausage.
Heat olive oil in a pan and sauté the onion, bell pepper, and sausage.
Add the boiled noodles and ketchup, then stir-fry together.
Season with salt and pepper to taste.
Top with mozzarella cheese, cover with a lid, and melt the cheese.