1. Cook the spaghetti in a large pot with plenty of salted water until al dente.
2. Meanwhile, heat some olive oil in a pan and fry the bacon or guanciale until crispy.
3. Add the finely chopped onion and sauté until translucent and softened.
4. Add the chopped tomatoes to the pan and simmer over medium heat for about 10 minutes to allow the flavors to blend.
5. Add the cooked spaghetti directly from the pot to the sauce and mix well to ensure the pasta absorbs the sauce. 6. Finally, sprinkle with freshly grated Parmesan cheese and pepper, and add a little salt to taste. Serve immediately and enjoy!