Bring a large pot of salted water to a boil. Cook rigatoni until al dente according to package directions. Drain and set aside.
- Heat olive oil in a large skillet over medium heat. Add chopped onion and sauté until softened.
- Add ground beef, cooking until browned. Drain excess fat and season with salt and pepper.
- Stir in minced garlic and cook for 1–2 minutes until fragrant.
- Reduce heat to low and stir in heavy cream. Bring to a gentle simmer, then add cream cheese. Whisk until melted and smooth.
- Stir in Parmesan cheese and half the mozzarella until fully melted into the sauce.
- Add cooked rigatoni and toss to coat evenly. If skillet is too small, mix in a large bowl or baking dish.
- Sprinkle remaining mozzarella on top. Cover until melted, or broil for 2–3 minutes until bubbly and golden.
- Garnish with parsley and red pepper flakes. Serve hot.