01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or a thin layer of oil.
02 - Boil the jumbo pasta shells in salted water according to package directions until al dente. Drain thoroughly and arrange on a clean surface to cool slightly for easier handling.
03 - Heat a large skillet over medium heat. Add the ground beef and cook, breaking it apart with a spoon, until browned and cooked through, about 6-8 minutes. Drain excess fat if needed.
04 - Add the diced onion and minced garlic to the skillet with the beef. Sauté until the onion is softened and fragrant, about 2-3 minutes. Season with Italian herbs, salt, and pepper. Remove from heat and let cool for 5 minutes.
05 - In a large bowl, combine ricotta cheese, egg, 1/2 cup grated Parmesan, 1 cup shredded mozzarella, and chopped parsley. Mix until well blended and smooth.
06 - Fold the cooled beef mixture into the ricotta cheese mixture until evenly distributed. The filling should be thick and scoopable.
07 - In a separate bowl, whisk together the marinara sauce and heavy cream until smooth and uniform.
08 - Spread half of the creamy tomato sauce mixture evenly across the bottom of the prepared baking dish.
09 - Fill each cooked pasta shell with approximately 2 tablespoons of the beef-ricotta filling. Place the stuffed shells in the baking dish, opening side up, arranging them in a single layer.
10 - Pour the remaining sauce evenly over the stuffed shells. Sprinkle with the remaining 1/2 cup shredded mozzarella and 2 tablespoons grated Parmesan cheese.
11 - Cover the baking dish tightly with aluminum foil. Bake for 25 minutes to heat through and allow flavors to meld.
12 - Remove the foil and return the dish to the oven. Bake uncovered for an additional 10 minutes, or until the cheese is bubbly and lightly golden brown.
13 - Let the stuffed shells rest for 5 minutes before serving to allow the filling to set. Garnish with additional fresh parsley if desired.