Preheat oven to 375°F (190°C).
Cook jumbo shells in a large pot of salted boiling water according to package directions for al dente, minus 2 minutes; drain and rinse with cold water to prevent sticking.
In a large skillet over medium-high heat, brown the ground beef with the diced onion until meat is cooked through; drain excess grease.
Add minced garlic to the beef and cook for 1 minute until fragrant; remove from heat and let cool slightly.
In a medium bowl, combine ricotta cheese, beaten egg, 1 cup of mozzarella cheese, parmesan cheese, Italian seasoning, salt, pepper, and the cooked beef mixture.
Spread 1 cup of marinara sauce into the bottom of a 9x13 inch baking dish.
Stuff each cooked shell generously with the beef and cheese mixture and arrange in the baking dish.
Top the shells with the remaining marinara sauce and the remaining 1 cup of mozzarella cheese.
Cover the dish with foil and bake for 20 minutes.
Remove foil and bake for an additional 10-15 minutes until the cheese is bubbly and lightly browned.
Garnish with fresh parsley before serving.