Preheat the oven to 375°F (190°C).
In a large pot, cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside.
In a large skillet over medium heat, cook the ground beef until browned, about 6-8 minutes. Add minced garlic and Italian seasoning. Season with salt and pepper, and cook for an additional 2 minutes. Remove from heat and let cool slightly.
In a mixing bowl, combine the cooked beef, ricotta cheese, half of the mozzarella cheese, Parmesan cheese, and egg. Mix well until fully combined.
Fill each cooked pasta shell with the beef and cheese mixture.
Spread 1 cup of marinara sauce evenly on the bottom of a 9x13-inch baking dish. Place the stuffed shells in the dish, seam side up.
Pour the remaining marinara sauce over the top of the shells and sprinkle the remaining mozzarella cheese over the sauce.
Cover the baking dish with aluminum foil and bake in the preheated oven for 20 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is bubbly and golden.
Let cool for a few minutes before serving. Garnish with fresh basil, if desired.