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 well and arrange on a clean surface to cool slightly.
03 - Brown ground beef in a large skillet over medium heat. Add diced onion and minced garlic; sauté until softened, about 2-3 minutes. Season with Italian herbs, salt, and black pepper. Remove from heat and cool for 5 minutes.
04 - Combine ricotta cheese, egg, grated Parmesan, 1 cup shredded mozzarella, and chopped parsley in a large bowl. Mix until evenly incorporated.
05 - Fold the cooled beef mixture into the ricotta filling until thoroughly blended.
06 - Whisk marinara sauce and heavy cream together in a separate bowl until smooth. Spread half the sauce mixture evenly across the bottom of the prepared baking dish.
07 - Fill each pasta shell with approximately 2 tablespoons of the beef-ricotta mixture. Arrange stuffed shells in a single layer in the baking dish, opening side facing up.
08 - Pour remaining sauce mixture evenly over the stuffed shells. Sprinkle with 1/2 cup mozzarella cheese and 2 tablespoons Parmesan cheese.
09 - Cover baking dish tightly with aluminum foil. Bake for 25 minutes.
10 - Remove foil and continue baking for 10 minutes, or until cheese is melted, bubbly, and lightly golden.
11 - Let stuffed shells rest for 5 minutes before serving to allow filling to set. Garnish with additional fresh parsley if desired.