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 according to package directions until al dente. Drain well and arrange on a clean surface to cool slightly for easier handling.
03 - Brown ground beef in a large skillet over medium heat, breaking 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 cool for 5 minutes.
04 - Combine ricotta cheese, egg, Parmesan, 1 cup mozzarella, and fresh parsley in a large bowl. Mix until fully incorporated, then fold in the cooled beef mixture.
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 shell with approximately 2 tablespoons of the beef-ricotta mixture. Place stuffed shells in a single layer, opening side up, in the baking dish over the sauce.
07 - Pour remaining sauce mixture evenly over the stuffed shells. Sprinkle with 1/2 cup mozzarella and 2 tablespoons Parmesan cheese.
08 - Cover baking dish tightly with aluminum foil. Bake at 375°F for 25 minutes to heat through and meld flavors.
09 - Remove foil and continue baking for 10 minutes, or until cheese is bubbly and lightly golden on top.
10 - Let stuffed shells rest for 5 minutes before serving to allow filling to set. Garnish with additional fresh parsley if desired.