Bring a large pot of salted water to a rolling boil over high heat. Add rigatoni, reduce to medium and cook to al dente according to package directions.
Meanwhile, melt butter in a large skillet. Add onion and cook until softened and beginning to brown.
Stir in garlic, salt, Italian seasoning, crushed red pepper flakes and pepper. Cook 1 minute but do not let the garlic burn.
Stir in flour until no white remains.
Add cream and tomato sauce and stir until combined. Continue simmering over medium heat until slightly thickened.
When pasta is cooked, reserve 1 cup pasta water and drain.
Add cooked pasta to finished sauce in pan, and stir in just enough pasta water so that the sauce comes together (start with ¼ cup and add more as needed).
Sprinkle with Parmesan cheese before serving.