Slice onion, bell pepper, and ham into bite-sized pieces.
Boil the spaghetti noodles and set aside.
Melt butter in a pan and sauté onion, bell pepper, and ham.
Add tomato ketchup and stir-fry the ingredients.
Add the boiled spaghetti noodles to the pan and mix while stir-frying.
Season with salt and pepper.
Add cheese and stir-fry until melted.
Plate the finished Neapolitan.