01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or a thin layer of olive oil.
02 - Boil the jumbo pasta shells in salted water until al dente according to package directions. Drain thoroughly and arrange on a clean surface to cool slightly for easier handling.
03 - Brown the ground beef in a large skillet over medium heat, breaking it apart with a wooden spoon. Add the diced onion and minced garlic, sautéing for 2-3 minutes until softened. Season with Italian herbs, salt, and pepper. Remove from heat and allow to cool for 5 minutes.
04 - In a large mixing bowl, combine ricotta cheese, egg, Parmesan, 1 cup mozzarella, and fresh parsley. Gently fold in the cooled beef mixture until evenly distributed.
05 - Whisk together marinara sauce and heavy cream in a separate bowl until smooth. Spread half of this sauce mixture evenly across the bottom of the prepared baking dish.
06 - Fill each cooked shell with approximately 2 tablespoons of the beef-ricotta filling. Place the stuffed shells in a single layer, opening side up, in the baking dish over the sauce.
07 - Pour the remaining creamy sauce over the stuffed shells, covering them evenly. Sprinkle the top with 1/2 cup mozzarella and 2 tablespoons Parmesan cheese.
08 - Cover the baking dish tightly with aluminum foil. Bake for 25 minutes to allow flavors to meld and pasta to heat through completely.
09 - Remove the foil and continue baking for 10 additional minutes, or until the cheese topping is bubbly and lightly golden brown.
10 - Let the stuffed shells rest for 5 minutes before serving to allow the filling to set. Garnish with additional fresh parsley if desired.