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 according to package directions until al dente. Drain thoroughly 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 for 2-3 minutes until softened. 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 chopped parsley in a large bowl. Mix until evenly incorporated. Fold in the cooled beef mixture until well blended.
05 - Whisk together marinara sauce and heavy cream in a separate bowl until smooth. Spread half the sauce mixture evenly across the bottom of the prepared baking dish.
06 - Fill each cooked shell with approximately 2 tablespoons of the beef-ricotta filling. Place stuffed shells open-side up in a single layer in the baking dish over the sauce.
07 - Pour the remaining creamy sauce 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 for 25 minutes to heat through and allow flavors to meld.
09 - Remove foil and return to oven for 10 minutes, or until cheese is bubbly and golden brown. Remove from oven and let rest for 5 minutes before serving.