Begin by boiling a large pot of salted water. Add the jumbo shells and cook according to the package instructions, typically about 9-11 minutes, until al dente. Once cooked, drain the shells and rinse them under cold water to stop the cooking process. Set them aside to cool while you prepare the meat sauce and filling.

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Once hot, add 1 pound of ground beef, seasoning it lightly with salt and pepper. Cook for 7-10 minutes, breaking up the meat with a wooden spoon until browned and cooked through.

Add 1 chopped onion and 2 minced garlic cloves to the skillet with the browned beef. Sauté for 3-5 minutes until the onion is softened and translucent.

Pour in 2 cups of marinara sauce and stir everything together. Increase the heat until the sauce comes to a gentle boil. Then lower the heat to a simmer and let it cook uncovered for 15 minutes.

In a mixing bowl, combine 15 ounces of ricotta cheese, 1 large egg, 1/2 cup of grated Parmesan cheese, and 1/4 cup of chopped fresh basil. Mix until all the ingredients are well incorporated and have a creamy texture.

Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish with cooking spray or olive oil. Spoon half of the prepared meat sauce evenly across the bottom of the dish.

Using a spoon, fill each cooled jumbo shell with the ricotta mixture, packing it gently. Arrange the stuffed shells on top of the meat sauce in your baking dish.

Sprinkle 1-2 cups of shredded mozzarella cheese evenly over the top of the assembled shells. Cover the baking dish with aluminum foil.

Place the covered baking dish into the preheated oven and bake for 15 minutes. Carefully remove the foil and allow the Stuffed Shells with Ground Beef to bake uncovered for an additional 10-15 minutes.

Once baked, remove the dish from the oven and let it cool for about 5-10 minutes. Garnish with extra fresh basil if desired, then serve warm.