01 - Preheat oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or a thin layer of oil.
02 - Boil 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 - Brown ground beef in a large skillet over medium heat, breaking it apart as it cooks. Add diced onion and minced garlic; sauté until softened, about 2-3 minutes. Season with Italian herbs, salt, and pepper. Remove from heat and allow to cool for 5 minutes.
04 - Combine ricotta cheese, egg, 1/2 cup Parmesan, 1 cup mozzarella, and chopped parsley in a large mixing bowl. Gently fold in the cooled beef mixture until evenly incorporated.
05 - Whisk together marinara sauce and heavy cream in a separate bowl until smooth and uniform.
06 - Spread half the sauce mixture evenly across the bottom of the prepared baking dish. Fill each pasta shell with approximately 2 tablespoons of the beef-ricotta filling. Arrange shells in the dish, opening side up.
07 - Pour the remaining sauce mixture over the stuffed shells, covering evenly. Sprinkle with 1/2 cup mozzarella and 2 tablespoons Parmesan cheese.
08 - Cover baking dish tightly with aluminum foil. Bake for 25 minutes at 375°F.
09 - Remove foil and continue baking uncovered for 10 minutes, or until cheese is bubbly and lightly golden brown.
10 - Allow stuffed shells to rest for 5 minutes before serving to set. Garnish with additional fresh parsley if desired.