Cut sausages diagonally into quarters, slice onion thinly, and slice bell pepper into thin strips.

Boil water in a pot and cook spaghetti according to package instructions.

Heat salad oil in a frying pan over medium heat, add sausages and vegetables, and sauté until the onion softens. Add ketchup and a bit of spaghetti cooking water, then mix in the cooked spaghetti.

For the salad, julienne the carrot and sprinkle with salt, then massage and let sit for 5 minutes. Squeeze out excess water. Slice the apple thinly and mix with lemon juice and sugar in a bowl.