Preheat Oven : Preheat oven to 375°F (190°C). Lightly grease a 9x13-inch baking dish.
Cook Pasta Shells : Boil jumbo pasta shells according to package directions until al dente. Drain and rinse with cold water to prevent sticking.
Prepare the Filling : In a large bowl, combine ricotta cheese, 1 cup mozzarella, Parmesan, sautéed spinach, egg, garlic powder, salt, pepper, and oregano. Mix until well combined.
Stuff the Shells : Spoon the cheese and spinach mixture into each cooked shell, filling generously.
Assemble in Baking Dish : Spread 1 cup of marinara sauce evenly on the bottom of the baking dish. Place stuffed shells in a single layer over the sauce. Pour remaining marinara sauce over the shells and sprinkle with remaining mozzarella.
Bake : Cover with foil and bake for 20 minutes. Remove foil and bake an additional 10–15 minutes until cheese is melted and bubbly.
Serve : Let cool for 5 minutes before serving. Garnish with fresh basil if desired.