01 - Bring a large pot of salted water to a boil. Add pasta and cook according to package directions until al dente. Drain, reserving ½ cup of the pasta water.
02 - Heat olive oil in a large skillet over medium heat. Add chopped onion and sauté for 3 to 4 minutes until soft and translucent.
03 - Stir in minced garlic and cook for 1 minute until fragrant.
04 - Add tomato paste and cook, stirring, for 1 minute to develop flavor.
05 - Pour in crushed tomatoes along with sugar, salt, black pepper, and red pepper flakes if using. Simmer gently for 8 to 10 minutes, stirring occasionally.
06 - Reduce heat, then stir in heavy cream and simmer for 2 to 3 minutes until sauce thickens slightly.
07 - Add drained pasta to the sauce along with Parmesan, butter, and half of the basil leaves. Toss well to coat, adding reserved pasta water gradually if sauce is too thick.
08 - Plate immediately, topped with remaining Parmesan and fresh basil leaves.