Preheat oven to 375 degrees Fahrenheit (190 degrees Celsius).
Cook jumbo shells in a large pot of boiling salted water according to package directions until slightly under-al-dente; drain and set aside.
In a large skillet over medium-high heat, brown the ground beef with the onion until fully cooked. Drain excess fat.
Stir in the minced garlic and cook for 1 minute. Remove from heat and allow to cool slightly.
In a medium bowl, combine the cooked beef mixture, ricotta cheese, beaten egg, Parmesan cheese, 1 cup of mozzarella cheese, Italian seasoning, salt, and pepper.
In a separate bowl, whisk together the marinara sauce and heavy cream to create a creamy sauce.
Spread 1 cup of the creamy sauce onto the bottom of a 9x13 inch baking dish.
Fill each cooked shell with approximately 2 tablespoons of the beef and ricotta mixture and place them in the baking dish.
Pour the remaining sauce over the stuffed shells and sprinkle with the remaining 1 cup of mozzarella cheese.
Cover with foil and bake for 20 minutes. Remove foil and bake for an additional 15 minutes until the cheese is melted and bubbling.
Garnish with fresh parsley before serving.