Preheat oven to 375 degrees Fahrenheit.
Cook jumbo shells in boiling salted water until al dente, drain, and rinse with cold water.
In a large skillet over medium heat, brown the ground beef until cooked through; drain excess fat.
Add onion and garlic to the beef and sauté until translucent, about 4 minutes.
Stir in the marinara sauce and simmer for 5 minutes.
Spread 1 cup of the meat sauce onto the bottom of a 9x13 inch baking dish.
In a medium bowl, combine ricotta, 1 cup of mozzarella, 1/4 cup of parmesan, egg, parsley, salt, and pepper.
Fill each cooked shell with the cheese mixture and place in the baking dish.
Spoon the remaining meat sauce over the shells and sprinkle with the remaining mozzarella and parmesan.
Cover with foil and bake for 20 minutes, then remove foil and bake for another 10 to 15 minutes until the cheese is melted and bubbling.