375°F (190°C). Grease a 9x13-inch baking dish with a little olive oil or cooking spray.

Bring a large pot of water with salt to a boil. Add the jumbo pasta shells and cook according to the package directions, usually about 9-11 minutes, until al dente. Drain the cooked shells and lay them on a sheet of parchment paper or a clean kitchen towel to cool and prevent them from sticking together.

In a large skillet, heat a little olive oil over medium-high heat. Add the ground beef and cook until browned, breaking it up with a spoon as it cooks (about 6-7 minutes).

Add the chopped onion and minced garlic, cooking for another 2-3 minutes until softened and fragrant.

Remove from heat and let it cool slightly.

In a medium bowl, combine the ricotta cheese, 1 cup of mozzarella, Parmesan cheese, egg, dried basil, oregano, salt, and pepper. Stir well until everything is evenly mixed.

Carefully spoon the beef and cheese mixture into each cooked shell. You can fill them generously without overstuffing. Place each stuffed shell into the prepared baking dish.

Once all the shells are filled, pour the marinara sauce evenly over the stuffed shells. Sprinkle the remaining ½ cup of shredded cheese on top of the sauce.

Cover the baking pan with aluminum foil (to prevent the cheese from browning too quickly) and bake in the preheated oven for 25-30 minutes. Remove the foil and bake for an additional baking time of 10-15 minutes, or until the cheese is melted and bubbly, and the sauce is hot.

Let the stuffed shells rest for about 5 minutes before serving. Garnish with fresh basil or parsley on top of the shells.