Preheat oven to 375°F (190°C).
Cook jumbo shells in a large pot of boiling salted water until al dente, drain, and set aside on parchment paper to prevent sticking.
In a large skillet over medium-high heat, brown the ground beef and onion until meat is cooked through; drain excess grease.
Add minced garlic to the skillet and cook for 1 minute until fragrant.
Stir in the marinara sauce and simmer on low for 5 minutes.
Spread 1 cup of the meat sauce onto the bottom of a 9x13 inch baking dish.
In a medium mixing bowl, combine ricotta cheese, 1 cup of mozzarella, Parmesan, egg, Italian seasoning, salt, and pepper until well incorporated.
Stuff each cooked shell with approximately 2 tablespoons of the cheese mixture and place into the baking dish.
Spoon the remaining meat sauce over the shells and sprinkle with the remaining 1 cup of mozzarella.
Cover the dish tightly with aluminum foil and bake for 20 minutes.
Remove the foil and bake for an additional 15 minutes until the cheese is melted, bubbly, and slightly browned.
Garnish with fresh parsley and allow the dish to rest for 5 minutes before serving.