Bring a large pot of salted water to a boil (using about 1 tablespoon of salt per 4 quarts of water). Cook the rigatoni according to package instructions until al dente. Reserve 1 cup of pasta water before draining and set the pasta aside.

Heat olive oil in a large 12 to 14-inch deep skillet over medium heat. Add diced onion and cook for 3-4 minutes until softened. Stir in the garlic and cook for an additional minute until fragrant.

Add ground beef to the skillet, breaking it apart with a spatula into small crumbles. Cook until browned and fully cooked through with no pink remaining, about 6-8 minutes. Drain any excess grease from the skillet.

Stir in the tomato paste, Italian seasoning, red pepper flakes (if using), and a pinch of salt and pepper. Cook for 2-3 minutes to enhance the flavor of the tomato paste and remove the raw taste.

Pour in the beef broth and heavy cream, stirring to combine. Simmer for 5-7 minutes, allowing the flavors to meld and the sauce to thicken slightly.

Gradually stir in the Parmesan cheese, adding a little at a time and stirring constantly until fully melted and incorporated into the sauce. Add mozzarella for extra creaminess, stirring until smooth.

Add the cooked rigatoni to the skillet and toss until well coated with the sauce. Use reserved pasta water, 2 to 3 tablespoons at a time, as needed to achieve the desired consistency.

Transfer the creamy beef rigatoni to serving plates and garnish with fresh parsley or basil. Serve hot with additional Parmesan on the side.