Heat olive oil in a large skillet or Dutch oven over medium-high heat. Add ground beef and cook, breaking it apart with a spoon, until browned. Drain off any excess fat.

Add chopped onion to the skillet with the beef and cook until softened, about 5-7 minutes. Stir in minced garlic and cook for another minute until fragrant.

Stir in crushed tomatoes, tomato sauce, beef broth, Italian seasoning, salt, black pepper, and red pepper flakes (if using). Bring the mixture to a simmer, then reduce heat to low, cover, and let it simmer for at least 15-20 minutes to allow flavors to meld, stirring occasionally.

While the sauce simmers, cook the pasta: Bring a large pot of salted water to a rolling boil. Add rigatoni or ziti and cook according to package directions until al dente (about 1 minute less than fully cooked). Drain well.

Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

Finish the sauce: Remove the beef sauce from heat and stir in the heavy cream until well combined.

Assemble the casserole: In a large mixing bowl, combine the drained pasta and the beef sauce. Stir gently until the pasta is evenly coated.

Pour half of the pasta mixture into the prepared baking dish. Sprinkle half of the shredded mozzarella cheese over the pasta.

Pour the remaining pasta mixture over the cheese layer. Top evenly with the remaining mozzarella cheese and sprinkle with grated Parmesan cheese.

Bake for 25-30 minutes, or until the casserole is bubbling around the edges and the cheese topping is melted and golden brown.

Let the casserole rest for 5-10 minutes before serving. Garnish with fresh chopped parsley, if desired.