Bring a large pot of salted water to a boil. Cook the jumbo shells according to package directions until al dente. Drain and rinse with cool water to prevent sticking.

In a large skillet over medium-high heat, cook the Italian sausage until browned, breaking it apart with a spoon. Add the onion and cook until soft, about 5 minutes. Stir in the garlic and cook for 1 more minute. Drain any excess fat.

In a large bowl, mix together the cooked sausage mixture, ricotta cheese, 1 cup of mozzarella cheese, Parmesan cheese, the beaten egg, and chopped parsley until well combined.

Preheat your oven to 375°F (190°C). Pour half of the marinara sauce into the bottom of a 9x13-inch baking dish. Fill each cooked shell with the sausage and cheese mixture and arrange them in the dish.

Top the shells with the remaining marinara sauce and the remaining 1 cup of mozzarella cheese. Bake for 25-30 minutes, until the cheese is melted and bubbly. Let stand for 10 minutes before serving.