Cut pancetta bacon into very fine julienne strips. In a large skillet, saute the onion in oil over medium heat until it starts to become transparent. Add the pancetta and fry gently until the onion is golden brown and the pancetta transparent. Add the tomatoes, salt and pepper, and cook over high heat for 10 minutes. Meanwhile cook the pasta uncovered in boiling salted water, stirring occasionally with a plastic or wooden fork. For cooking time, see package instructions. When the pasta is ready, drain in a colander, then add to the sauce over low heat. Mix gently, top with cheese and serve immediately.