Boil the Pasta: Bring a large pot of salted water to a rolling boil. Carefully add the jumbo pasta shells, cooking them until al dente as per package instructions. Drain and rinse under cold water to stop cooking, then set aside.

Mix the Filling: In a large mixing bowl, combine ricotta cheese, chopped spinach, grated Parmesan cheese, half of the shredded mozzarella cheese, the egg, garlic powder, Italian seasoning, salt, and pepper. Mix until well combined.

Preheat the Oven: Preheat your oven to 375°F (190°C). Pour a thin layer of marinara sauce into the bottom of a 9x13-inch baking dish.

Stuff the Shells: Fill each cooked pasta shell with the spinach and ricotta mixture. Place the filled shells seam side up in the baking dish.

Layer and Bake: Pour the remaining marinara sauce over the stuffed shells, sprinkle the rest of the mozzarella cheese on top. Cover with foil and bake for 20 minutes. Remove foil and bake for an additional 10–15 minutes.

Cool and Serve: Let the stuffed shells cool for a few minutes before serving. Garnish with fresh basil or extra Parmesan cheese if desired.