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 carefully and arrange on a clean surface to cool slightly.
03 - In a large skillet over medium heat, brown the ground beef, breaking it apart as it cooks. Add the diced onion and minced garlic; sauté until softened, approximately 2-3 minutes. Season with Italian herbs, salt, and black pepper. Remove from heat and allow to cool for 5 minutes.
04 - In a large bowl, combine ricotta cheese, egg, 1/2 cup Parmesan, 1 cup mozzarella, and chopped parsley. 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 and evenly combined.
06 - Spread half the sauce mixture across the bottom of the prepared baking dish. Fill each pasta shell with approximately 2 tablespoons of the beef-ricotta filling and place them open-side up in the dish, arranging in rows.
07 - Pour the remaining sauce evenly over all 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 additional minutes, or until the cheese is bubbly and lightly golden on top.
10 - Allow the stuffed shells to rest for 5 minutes before serving to set the filling. Garnish with additional fresh parsley if desired.