Bring a large pot of water, seasoned generously with salt, to a rolling boil. Add the tortellini and cook according to the package directions until just tender. Drain well and set aside.
Place a large skillet over medium heat. Add the ground beef and diced onion, cooking until the meat is fully browned and the onion has softened. Stir in the minced garlic and let cook for an additional minute. If needed, discard any excess fat from the pan.
Stir the marinara sauce into the meat mixture, followed by the Italian seasoning. Reduce heat and allow the sauce to gently simmer for 5 to 7 minutes, stirring occasionally to blend the flavors.
In a large mixing bowl, combine the drained tortellini with the meat sauce. Add half of the shredded mozzarella and gently mix until the ingredients are evenly distributed.
Lightly grease a 9x13-inch baking dish. Spread the tortellini mixture evenly in the dish. Sprinkle the remaining mozzarella over the top, followed by the grated Parmesan.
Preheat the oven to 375°F (190°C). Bake the casserole uncovered for 20 to 25 minutes, or until the cheese topping is completely melted and bubbling around the edges.
Remove from the oven and allow the dish to rest briefly before serving. If desired, garnish with a sprinkle of freshly chopped parsley for a bright finish.