Preheat your oven to 375°F (190°C).
Bring a large pot of salted water to a boil. Cook the jumbo pasta shells according to the package instructions until al dente, usually about 8-10 minutes. Drain and set aside to cool slightly.
In a large mixing bowl, combine the ricotta cheese, chopped spinach, mozzarella cheese, Parmesan cheese, egg, garlic powder, Italian seasoning, and a pinch of salt and pepper. Mix until well combined.
Carefully stuff each cooked pasta shell with the cheese and spinach mixture using a spoon.
Spread a thin layer of marinara sauce evenly across the bottom of the baking dish.
Place the stuffed shells in the baking dish, seam side up, in a single layer. Pour the remaining marinara sauce over the stuffed shells.
Cover the dish with aluminum foil and bake in the preheated oven for 20 minutes.
After 20 minutes, remove the foil and sprinkle any additional mozzarella cheese on top. Bake uncovered for another 10 minutes, or until the cheese is bubbly and golden.
Remove from the oven and let it cool for a few minutes before serving.