01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or oil.
02 - Boil jumbo pasta shells in salted water according to package directions until al dente. Drain well and arrange on a clean surface to cool slightly.
03 - Brown ground beef in a large skillet over medium heat, breaking it apart with a spoon. Add diced onion and minced garlic; sauté until softened, 2-3 minutes. Season with Italian herbs, salt, and pepper. Remove from heat and let cool for 5 minutes.
04 - Combine ricotta cheese, egg, Parmesan, 1 cup mozzarella, and chopped parsley in a large bowl. Mix until smooth. Fold in the cooled beef mixture until evenly incorporated.
05 - Whisk together marinara sauce and heavy cream in a separate bowl until fully combined.
06 - Spread half the creamy sauce across the bottom of the prepared baking dish. Fill each pasta shell with approximately 2 tablespoons of the beef-ricotta mixture. Place filled shells in a single layer in the dish.
07 - Pour the remaining sauce evenly over the stuffed shells. Sprinkle with 1/2 cup mozzarella and 2 tablespoons Parmesan cheese.
08 - Cover the baking dish tightly with aluminum foil. Bake for 25 minutes.
09 - Remove foil and continue baking for 10 minutes, or until cheese is bubbly and 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.