Slice the onion into 5mm strips and the bell peppers into 3mm rings. Remove seeds from the peppers.

Cut the sausages diagonally into 5mm slices.

Heat salad oil in a frying pan over medium heat. Sauté the onions until translucent, then add bell peppers, sausages, and mushrooms. Push the ingredients to one side of the pan.

Add ketchup to the empty side of the pan and cook until its moisture evaporates. Mix it with the other ingredients.

Boil spaghetti in salted water for 30 seconds less than the package instructions. Drain well.

Add the cooked spaghetti to the pan and mix well with the sauce and ingredients. Adjust the taste with salt and pepper.