Cook the large pasta shells according to package directions, then drain and set aside to cool slightly. This prevents them from becoming too soft when you're filling them later.

In a large skillet, heat olive oil over medium-high heat and add diced onion and minced garlic. Brown the ground beef for 8-10 minutes, breaking it into small pieces as it cooks until no pink remains, then drain excess fat.

In a large mixing bowl, combine ricotta cheese, mozzarella, parmesan, egg, Italian seasoning, salt, and black pepper. Stir until the mixture is smooth and well incorporated throughout.

Gently fold the cooked beef mixture into the cheese filling, then add fresh parsley and mix until everything is evenly distributed. Spread a thin layer of marinara sauce on the bottom of a 9x13 baking dish.

Carefully fill each pasta shell with the cheese and beef mixture using a small spoon, placing filled shells seam-side up in the prepared baking dish. Pour remaining marinara sauce over the shells and top with beef broth to keep everything moist.

Cover the dish with foil and bake at 375°F for 25 minutes, then remove foil and bake uncovered for 12-15 minutes until the golden tender stuffed shells recipe is bubbling and the cheese on top begins to brown slightly.

Remove from the oven and let rest for 5 minutes before serving to allow the filling to set slightly and make plating easier.