Cook the pasta shells in salted boiling water until al dente, about 8 to 10. Drain and rinse under cold water.
In a mixing bowl, combine ricotta cheese, chopped spinach, half of the mozzarella, Parmesan, beaten egg, garlic powder, Italian seasoning, salt, and pepper. Mix until well combined.
Stuff each pasta shell with the ricotta and spinach mixture and place in a baking dish, open side up.
Pour 1 cup of marinara sauce over the bottom of the baking dish, then place the stuffed shells and pour the remaining sauce on top. Sprinkle with the rest of the mozzarella.
Preheat oven to 375°F. Cover with foil and bake for 25. Remove foil and bake for an additional 10 to 15 until cheese is bubbly and golden brown.