Bring a large pot of salted water (about 1-2 tablespoons salt) to a boil. Cook the rigatoni according to package instructions until al dente, usually 10-12 minutes. Reserve 1 cup of pasta water before draining and set the pasta aside.

Heat olive oil in a large 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. Cook until browned and fully cooked through, about 6-8 minutes. Drain any excess grease from the skillet, leaving about 1-2 tablespoons.

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.

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 until fully melted and incorporated into the sauce. Add mozzarella for extra creaminess, stirring until smooth and glossy.

Add the cooked rigatoni to the skillet and toss until well coated with the sauce. Use reserved pasta water 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.