Bring a large pot of salted water to a boil and cook rigatoni until al dente. Drain and set aside.
- In a large skillet, heat olive oil over medium heat. Add ground beef and cook until browned. Season with salt, pepper, and optional red pepper flakes. Remove from skillet and set aside.
- In the same skillet, add onion and sauté until softened. Stir in garlic and cook for 1–2 minutes until fragrant.
- Reduce heat and pour in heavy cream. Stir in cream cheese and Parmesan, whisking until smooth and slightly thickened (3–4 minutes).
- Return beef to skillet, then add drained rigatoni. Toss well to coat pasta in sauce. Stir in half the shredded mozzarella.
- Sprinkle remaining mozzarella on top. Cover for a few minutes or broil briefly until cheese melts and bubbles.
- Garnish with fresh parsley and serve hot.