Start by boiling a large pot of water and add a generous amount of salt. Cook the jumbo pasta shells according to the package instructions until al dente, usually about 10-12 minutes. Once cooked, drain the pasta in a colander and run cold water over them gently to stop the cooking process. Set the shells aside to cool down while you prepare the filling.

In a large skillet over medium heat, add about a pound of ground beef. Cook until browned, breaking it apart with a spatula while stirring, which usually takes around 7-10 minutes. Season the meat with salt, pepper, and a sprinkle of Italian seasoning. Once fully cooked, drain the excess fat and let the filling cool slightly before mixing in the cheeses.

In a spacious mixing bowl, combine the ricotta cheese with half of the shredded mozzarella, grated Parmesan, and garlic powder. Stir this mixture until it's smooth and well combined. Once the ground beef has cooled sufficiently, fold it into the cheese mixture until fully integrated.

Preheat your oven to 350°F (175°C) while you prepare to fill the pasta shells. Spread a thin layer of marinara sauce on the bottom of a baking dish. Take a cooled shell and stuff it generously with the cheese and beef filling using a spoon or your hands, then place it seam side up in the dish. Repeat this process until all shells are filled and arranged neatly in the dish.

Once all your shells are assembled, pour the remaining marinara sauce over the stuffed shells, ensuring they are well coated. Top everything generously with the remaining mozzarella cheese. Cover the baking dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes until the cheese is bubbling and golden.

Once your Best Stuffed Shells with Ground Beef are out of the oven, let them cool for a few minutes to set. Serve the shells hot, garnished with fresh parsley if desired.