Cook spaghetti in boiling water with salt according to the package.

Cut sausage into bite size pieces and slice onion and green pepper thinly.

Heat a frying pan at medium high heat and add oil. Add sausage pieces, onion and peppers, and cook for a minute or two. Add cooked spaghetti and stir-fry for a minute, then add ketchup. Season with a little salt.

Divide pasta in bowls and top with parsley and cheese if you wish.