01 - Bring a large pot of salted water to boil. Cook pasta according to package directions until al dente. Reserve 1/2 cup pasta water, drain well.
02 - Heat olive oil in a large skillet over medium heat. Add onion and sauté 3–4 minutes until translucent. Add garlic and cook 30 seconds until fragrant.
03 - Pour in crushed tomatoes, sugar, salt, black pepper, and red pepper flakes if desired. Simmer 8–10 minutes, stirring occasionally, until slightly thickened.
04 - Reduce heat to low. Stir in heavy cream and Parmesan. Cook 2–3 minutes, stirring constantly, until sauce is smooth and well combined.
05 - Add drained pasta to the skillet. Toss to coat completely, adding reserved pasta water as needed to achieve desired consistency.
06 - Remove from heat. Fold in fresh basil leaves. Serve immediately with extra Parmesan and additional basil garnish.