01 - Preheat 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 per package directions. 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 2-3 minutes until softened. Season with Italian herbs, salt, and pepper. Remove from heat and let cool 5 minutes.
04 - Combine ricotta cheese, egg, Parmesan, 1 cup mozzarella, and chopped parsley in a large bowl. Mix until smooth. Fold in cooled beef mixture until evenly distributed.
05 - Whisk together marinara sauce and heavy cream in a separate bowl until fully incorporated.
06 - Spread half the creamy sauce mixture across the bottom of the prepared baking dish.
07 - Fill each cooked shell with approximately 2 tablespoons of the beef-ricotta filling. Arrange filled shells in a single layer in the baking dish, opening side up.
08 - Pour remaining sauce evenly over stuffed shells. Sprinkle with 1/2 cup mozzarella and 2 tablespoons Parmesan cheese.
09 - Cover baking dish tightly with aluminum foil. Bake at 375°F for 25 minutes.
10 - Remove foil and continue baking 10 additional minutes until cheese is melted, bubbly, and lightly golden. Let rest 5 minutes before serving. Garnish with additional parsley if desired.