01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or olive oil.
02 - Boil jumbo pasta shells in salted water until al dente according to package directions. Drain thoroughly and arrange on a clean surface to cool slightly.
03 - Brown ground beef in a large skillet over medium heat, breaking it apart as it cooks. Add diced onion and minced garlic; sauté 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, grated Parmesan, 1 cup shredded mozzarella, and chopped parsley in a large bowl. Mix until well blended. 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 pasta shell with approximately 2 tablespoons of the beef-ricotta filling. Place stuffed shells in the baking dish, opening side facing up, arranging them in a single layer.
07 - Pour the remaining creamy sauce over the stuffed shells, covering them evenly. Sprinkle with 1/2 cup shredded mozzarella and 2 tablespoons grated Parmesan cheese.
08 - Cover the baking dish tightly with aluminum foil. Bake for 25 minutes to allow flavors to meld and shells to heat through.
09 - Remove foil and continue baking uncovered for 10 minutes until cheese is bubbly and golden brown on top.
10 - Let the stuffed shells rest for 5 minutes before serving to allow filling to set. Garnish with additional fresh parsley if desired.