Boil a large pot of salted water and cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside to cool slightly.

In a skillet over medium heat, brown the ground beef until fully cooked. Drain excess fat, then add marinara sauce, salt, pepper, and Italian seasoning. Stir and let simmer for about 5 minutes.

In a large bowl, combine ricotta cheese, egg, half of the mozzarella, and half of the Parmesan cheese. Mix until well incorporated.

Carefully spoon a generous amount of the cheese mixture into each cooled pasta shell, followed by the ground beef mixture.

Spread a thin layer of marinara on the bottom of a baking dish. Place the stuffed shells upright in the dish, cover with remaining marinara, and sprinkle with remaining mozzarella and Parmesan cheese.

Preheat the oven to 350°F (175°C). Cover the dish with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 5-10 minutes until the cheese is bubbly and golden.

Let cool for a few minutes before sprinkling with fresh parsley and serving.