Preheat the oven to 350°F (180°C).
Cook the jumbo pasta shells according to the package instructions until al dente. Drain the shells and set aside.
In a large skillet over medium-high heat, cook the ground beef until browned and cooked through. Drain any excess fat from the skillet.
In a medium bowl, combine the ricotta cheese, 1 cup of the shredded mozzarella cheese, ½ cup grated Parmesan cheese, beaten egg, chopped fresh parsley, minced garlic, salt, and black pepper. Mix well.
Add the cooked ground beef to the cheese mixture and stir to combine.
Spread a thin layer of marinara sauce in the bottom of a 9x13 inch baking dish.
Using a spoon, stuff each cooked jumbo pasta shell with the beef and cheese mixture. Place the stuffed shells in the baking dish.
Pour the remaining marinara sauce over the top of the stuffed shells, making sure to cover all of the shells. Sprinkle the remaining 1 cup of shredded mozzarella cheese and ½ cup of grated Parmesan cheese evenly over the shells.
Cover the baking dish with foil and bake for 25 minutes.
Remove the foil from the baking dish and continue baking for an additional 10-15 minutes, until the cheese is melted and bubbly.
Let the stuffed shells cool for a few minutes before sprinkling with extra chopped parsley and serving.