Preheat the oven to 375°F (190°C).
Cook the jumbo pasta shells in a large pot of salted boiling water according to package instructions, until al dente. Drain and set aside.
In a skillet, heat the olive oil over medium heat. Add the minced garlic and ground beef, cooking until the beef is browned. Season with salt and black pepper. Drain excess fat if necessary.
In a mixing bowl, combine the cooked ground beef, ricotta cheese, half of the mozzarella cheese, Parmesan cheese, egg, and chopped parsley. Mix well until everything is combined.
Spread one cup of marinara sauce evenly across the bottom of a baking dish.
Stuff each cooked shell with the beef and ricotta mixture, and place them seam side up in the baking dish.
Pour the remaining marinara sauce over the stuffed shells, then sprinkle the remaining mozzarella cheese on top.
Cover the baking dish with foil and bake for 25 minutes. Remove the foil and bake for an additional 5 minutes or until the cheese is bubbly and golden.
Allow to cool for a few minutes before serving.