Cook Pasta Al Dente: Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to the package instructions until just tender but still firm to the bite. Before draining, reserve about one cup of the starchy pasta water, then drain the pasta and set aside.
Sauté Aromatics and Spice: In a large skillet over medium-high heat, melt the butter. Add the diced onion and red chili flakes (if using). Cook, stirring occasionally, for about 5 to 7 minutes until the onion turns translucent and soft, releasing its natural sweetness.
Soften Fresh Tomatoes and Garlic: Add the diced tomatoes and minced garlic to the skillet. Cook for another 2 to 3 minutes, stirring frequently, until the tomatoes break down slightly and the garlic is fragrant but not browned.
Build the Creamy Tomato Sauce: Lower the heat to medium. Pour in the tomato sauce and heavy cream, stirring well to combine. Season with salt and pepper. Let the sauce gently simmer for about 5 minutes, allowing the flavors to meld and the sauce to thicken slightly.
Incorporate Cheese and Pasta: Stir in the grated Parmesan cheese until it melts smoothly into the sauce. Add the cooked pasta to the skillet, tossing gently to coat every piece thoroughly. If the sauce feels too thick, gradually add reserved pasta water a little at a time until you achieve a creamy, silky consistency.
Finish with Fresh Basil and Serve: Fold in the chopped fresh basil just before serving to preserve its bright flavor and aroma. Plate the pasta and garnish with additional Parmesan and basil leaves. Serve immediately for the best taste and texture.