In a large pot, heat up a bit of oil and cook the ground beef, breaking up and stirring. When browned and cooked through, remove from the pan.
Add the onion, celery, and carrot and cook, stirring until soft, but not browned.
Stir in the tomato paste, tomatoes, beef stock, cream, and ground beef. Simmer for 15-20 minutes. Taste and season with salt and pepper.
Heat the oven to 375°F. Cook the rigatoni 1-2 minutes shy of al dente according to the package instructions, then drain well. Arrange the pasta upright in a lightly oiled 8 inch springform pan.
Cut 1/2 of the mozzarella into strips that will fit the rigatoni and slice the remaining half. Add the mozzarella strips into random rigatoni (or you can stuff every single one).
Top with a generous layer of sauce and layer with the sliced mozzarella. Top with grated parmesan, if desired.
Bake for 30-40 minutes or until the cheese is melty and golden. Let rest for 5-10 minute before removing from the pan. Serve with the extra sauce on the side if desired. Enjoy!