Bring a large pot of salted water to a boil. Add the 12 large pasta shells and cook according to the package instructions until al dente, usually about 10-12 minutes. Drain carefully and set aside on a baking sheet to cool slightly.
While the pasta cooks, heat a large skillet over medium heat. Add the ground beef and cook, breaking it apart with a spatula, until browned and cooked through, about 6-8 minutes. Drain any excess fat, then season with garlic powder, onion powder, Italian seasoning, salt, and pepper. Stir well to combine and remove from heat.
In a mixing bowl, combine the ricotta cheese, shredded mozzarella, grated Parmesan, and the cooked ground beef. Mix until everything is well incorporated.
Carefully spoon the beef and cheese mixture into each pasta shell until all are generously filled.
Spread about 1 cup of marinara sauce in the bottom of your baking dish. Arrange the stuffed shells in the dish, open side up. Pour the remaining marinara sauce over the shells, making sure they’re well covered.
Preheat your oven to 375°F (190°C). Cover the baking dish with foil and bake for 25 minutes. Remove the foil and bake for an additional 10 minutes or until the cheese is bubbly and slightly golden on top.
Once out of the oven, let the pasta shells cool for a few minutes. Garnish with fresh basil leaves and serve warm.