In a Large, heavy skillet, heat the oil over a medium flame. Add the pancetta and sauté until golden brown, about 8 minutes. Add the onion and sauté until tender, about 5 minutes. Add the garlic and red pepper flakes and sauté until fragrant, about 30 seconds. Stir in the tomato purée, and ½ teaspoon each of salt and pepper. Simmer, uncovered, over medium-low heat until the sauce thickens slightly and the flavors blend, about 15 minutes. Stir in the cheese. Season with more salt and pepper to taste. (The sauce can be made 1 day ahead. Cool, then cover and refrigerate. Rewarm over medium heat before using.)