Slice the onion, bell pepper, sausage, and garlic thinly.
Boil the spaghetti noodles in boiling water.
Heat olive oil in a pan and sauté the garlic.
Add the onion, bell pepper, and sausage in order and stir-fry.
Add the boiled spaghetti and ketchup, mixing and stir-frying together.
Taste and season with salt and pepper.
Turn off the heat and top with cheese to melt.