Boil the spaghetti noodles.
Cut the onion, bell pepper, and sausage into bite-size pieces.
Heat oil in a pan and stir-fry the onion, bell pepper, and sausage.
Add ketchup, a little sugar, salt, and pepper, then stir-fry.
Add the boiled spaghetti noodles and stir-fry together with the ingredients.
Transfer to a plate to finish.