Preheat your oven to 350°F (175°C). Bring a large pot of salted water to a boil and cook the jumbo pasta shells according to the package instructions until they're al dente. Drain and set aside.

In a skillet, brown the ground beef over medium heat. Add garlic powder, Italian seasoning, salt, and pepper. Cook until no longer pink. Drain excess grease and cool for a few minutes.

In a bowl, combine ricotta, half of the mozzarella, and half of the Parmesan cheeses. Stir in the cooled beef mixture until fully blended.

Stuff each pasta shell with the beef and cheese mixture.

Spread marinara sauce at the bottom of a baking dish. Place stuffed shells seam-side-up and pour remaining marinara over them. Top with the rest of the mozzarella and Parmesan. Cover with foil and bake for about 25 minutes. Remove foil and bake for an additional 15 minutes until bubbly and golden.

Allow the shells to cool slightly. Garnish with fresh parsley and serve.