Preheat your oven to 375°F (190°C).
Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside to cool slightly.
In a large bowl, mix together the ricotta cheese, 1/2 cup mozzarella cheese, grated Parmesan cheese, Italian seasoning, garlic powder, salt, pepper, and the egg until well combined.
Spread 1 cup of marinara sauce over the bottom of a 9x13-inch baking dish.
Using a spoon, fill each cooked pasta shell with the ricotta mixture, then place them in the baking dish on top of the marinara sauce.
Once all shells are filled and arranged in the dish, pour the remaining marinara sauce over the top of the shells.
Sprinkle the remaining 1/2 cup of mozzarella cheese on top.
Cover the dish with aluminum foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is melted and bubbly.
Allow to cool for a few minutes before serving. Enjoy!