01 - Preheat the oven to 375°F. Lightly grease a 9x13-inch baking dish with cooking spray or olive oil.
02 - Boil the jumbo pasta shells in salted water according to package directions until al dente. Drain thoroughly and arrange on a clean surface to cool slightly.
03 - Brown the ground beef in a large skillet over medium heat, breaking it apart with a spoon. Add the diced onion and minced garlic; sauté until softened, 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, Parmesan, 1 cup mozzarella, and fresh parsley in a large bowl. Mix until smooth and well incorporated. Fold in the cooled beef mixture.
05 - Whisk together the 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 cooled pasta shell with approximately 2 tablespoons of the beef-ricotta filling. Place the stuffed shells in a single layer in the baking dish, opening side facing up.
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 to heat through and meld flavors.
09 - Remove the foil and continue baking for 10 minutes, or until the cheese is bubbly and lightly golden brown.
10 - Allow the stuffed shells to rest for 5 minutes before serving to set the filling. Garnish with additional fresh parsley if desired.