Preheat the oven to 375°F.
Cook the pasta shells according to package directions until almost al dente.
Drain and rinse with cool water to stop the cooking process.
Heat the olive oil in a large skillet over medium heat.
Add the ground beef and onion; cook until browned and the onion has softened. Drain any excess grease.
Stir in the garlic. Cook for 1 minute until fragrant. Pour in the marinara and stir.
Combine the ricotta cheese, egg, Parmesan cheese, and 1 cup of shredded mozzarella cheese in a large bowl.
Spread 1 cup of the meat sauce across the bottom of a greased 9x13-inch baking dish.
Fill each cooked shell with the ricotta cheese mixture and arrange the shells in the prepared dish.
Spoon the meat sauce over the top of the stuffed shells.
Top with the remaining mozzarella cheese.
Cover with foil and bake for 25 minutes.
Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and bubbly. You can also broil for a few minutes if you want the cheese extra browned.
Sprinkle with fresh parsley before serving.