Preheat your oven to 375°F and lightly grease a 9x13-inch baking dish.

Cook the jumbo pasta shells in a large pot of salted boiling water until just al dente, about 9 minutes. Drain and lay them out on a baking sheet so they don't stick together.

While the shells cook, brown the ground beef in a skillet over medium-high heat. Add the garlic and cook for another minute. Drain excess fat and let the mixture cool slightly.

In a large bowl, mix together the ricotta, 1 cup of mozzarella, Parmesan, egg, Italian seasoning, onion powder, salt, and pepper. Fold in the cooled beef mixture until everything is well combined.

Spread 1 cup of marinara sauce across the bottom of your prepared baking dish.

Spoon the beef and cheese filling generously into each shell and nestle them snugly into the dish, open side up.

Pour the remaining marinara sauce over the stuffed shells and top with the remaining ½ cup of mozzarella.

Cover with foil and bake for 30 minutes, then uncover and bake for another 10 minutes until the cheese is bubbly and golden.

Let the dish rest for 5 minutes before serving.