01 - Bring a large pot of salted water to a boil. Cook pasta 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 to the skillet, stirring continuously for 1 minute to intensify flavor.
05 - Pour in crushed tomatoes, sugar, salt, black pepper, and red pepper flakes if using. Simmer uncovered for 8 to 10 minutes, stirring occasionally.
06 - Reduce heat to low and stir in heavy cream. Simmer for 2 to 3 minutes until sauce thickens slightly.
07 - Add drained pasta to the sauce along with Parmesan cheese, butter, and half of the torn basil. Toss gently to coat, adding reserved pasta water gradually if sauce consistency is too thick.
08 - Plate immediately and garnish with remaining Parmesan cheese and basil leaves.