Bring a large pot of salted water to a boil. Cook rigatoni until al dente. Reserve 1/2 cup pasta water and drain.
Warm olive oil in a wide skillet over medium heat. Soften the chopped onion for 3 to 4 minutes.
Stir in garlic and red pepper flakes. Cook until fragrant, about 30 to 60 seconds.
Add tomato paste and cook, stirring, for 1 minute to slightly darken and caramelize.
Pour in crushed tomatoes. Season with salt, black pepper, and sugar. Simmer gently for 10 to 12 minutes.
Toss in drained rigatoni and splash in the reserved pasta water. Stir until the sauce clings to the pasta.
Fold in the chopped basil. Taste and adjust salt or heat as needed.
Serve hot with grated Parmesan on top.