Bring a large pot of salted water to a rolling boil. Add spaghetti and cook according to package instructions, about 8-10 minutes.

In a large pan, heat olive oil over medium heat. Add finely diced onion and sauté for 3-4 minutes until soft.

Add minced garlic and chopped chili pepper, stir and cook for 1 minute.

Stir in peeled tomatoes and simmer for 10-15 minutes until thickened.

Add drained spaghetti to the sauce, toss to coat. Add reserved pasta water if needed.

Serve in bowls topped with chopped parsley.