Preheat your oven to 375°F (190°C).
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, cook the ground beef until browned, breaking it apart with a spoon. Add the minced garlic and Italian seasoning during the last minute of cooking. Season with salt and pepper. Remove from heat and let cool.
In a mixing bowl, combine the cooked ground beef, ricotta cheese, half of the mozzarella cheese, grated Parmesan cheese, and large egg. Mix until well combined.
Spoon the beef and ricotta mixture into each pasta shell, filling them generously.
Spread half of the marinara sauce on the bottom of a 9x13 inch baking dish. Arrange the stuffed shells on top of the sauce, seam side up.
Pour the remaining marinara sauce over the stuffed shells and sprinkle with the remaining mozzarella cheese.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. Remove the foil and bake for an additional 15 minutes until cheese is bubbling and lightly golden.
Allow the dish to cool for a few minutes before serving. Garnish with fresh basil, if desired.