Bring a large pot of salted water to a boil and cook manicotti shells until al dente, about 8-10 minutes. Drain and rinse under cold water, then set aside to cool slightly.

Preheat the oven to 350°F (175°C).

Heat a sauté pan over medium heat and brown 1 lb ground beef until no longer pink, about 5-7 minutes. Drain excess fat.

In a mixing bowl, combine the browned beef with 15 oz ricotta cheese, half of the shredded mozzarella, and 1/2 cup grated Parmesan cheese.

Season the mixture with 1 tsp garlic powder, 1 tsp Italian seasoning, salt, and pepper. Stir until evenly combined.

Fill each manicotti shell with the meat and cheese mixture using a spoon or pastry bag.

Spread 1 cup of marinara sauce in the bottom of a large baking dish. Arrange the stuffed shells in the dish, then pour the remaining 1 cup of marinara sauce over the top and sprinkle with the remaining shredded mozzarella.

Cover the baking dish with aluminum foil and bake at 350°F (175°C) for 30 minutes. Remove the foil and bake for an additional 10 minutes until the cheese is golden and bubbly.