01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or 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.
03 - Brown the ground beef in a large skillet over medium heat, breaking it apart with a spoon. Add the diced onion and minced garlic; cook for 2-3 minutes until softened. Season with Italian herbs, salt, and black pepper. Remove from heat and allow to cool for 5 minutes.
04 - Combine ricotta cheese, egg, 1/2 cup Parmesan, 1 cup mozzarella, and fresh parsley in a large bowl. Mix until smooth. Fold in the cooled beef mixture until evenly incorporated.
05 - Whisk together the marinara sauce and heavy cream in a separate bowl until smooth and uniform.
06 - Spread half of the creamy sauce mixture over the bottom of the prepared baking dish. Fill each pasta shell with approximately 2 tablespoons of the beef-ricotta filling. Place the stuffed shells in the dish in a single layer, opening side up.
07 - Pour the remaining sauce evenly over all the stuffed shells. Sprinkle with the remaining 1/2 cup mozzarella cheese and 2 tablespoons Parmesan cheese.
08 - Cover the baking dish tightly with aluminum foil. Bake for 25 minutes in the preheated oven.
09 - Remove the foil and continue baking for 10 minutes, or until the cheese is bubbly and lightly golden. Remove from oven and let rest for 5 minutes before serving to allow the filling to set.