Preheat your oven to 350°F and set aside a 9x13 casserole dish.
Cook the pasta shells according to the package directions. Drain and set aside on a baking sheet to prevent the shells from sticking.
In a large skillet over medium-high heat, add the ground beef, onions, and garlic. Cook and crumble the ground beef until no longer pink, about 5 minutes.
Stir in the marinara sauce and Italian seasoning. Bring to a simmer, then remove from the heat.
In a medium bowl, mix together the ricotta, 1 ½ cups of mozzarella, parmesan, parsley, and egg. Set aside
Spread ⅔ of the meat sauce on the bottom of the casserole dish.
Fill each shell with 2 tablespoons of the ricotta filling and place in the pan. Spoon the remaining meat sauce over the shells, then top with the remaining mozzarella.
Cover with foil and bake for 30 minutes. Remove the foil and bake for another 15 minutes. Remove from the oven and garnish with extra parsley. Enjoy!