Preheat your oven to 375°F (190°C).
Cook the jumbo pasta shells according to the package instructions until al dente. Drain and set aside to cool.
In a large skillet over medium heat, add the ground beef. Cook until browned and no longer pink, breaking it up with a spoon as it cooks (about 7-10 minutes). Drain excess fat if necessary.
In a mixing bowl, combine the cooked beef, ricotta cheese, 1 cup of mozzarella cheese, Parmesan cheese, egg, garlic powder, onion powder, Italian seasoning, salt, and pepper. Mix well until fully incorporated.
Spoon a generous amount of the beef and ricotta mixture into each cooked pasta shell. Place the filled shells into a greased 9x13 inch baking dish, open side up.
Pour the marinara sauce evenly over the stuffed shells. Sprinkle any remaining mozzarella cheese on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
After 25 minutes, remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Remove from the oven and let it sit for a few minutes before serving. Garnish with fresh parsley, if desired.