Cook the Pasta: Fill a large pot with salted water and bring it to a boil. Add the jumbo pasta shells and cook as directed on the package until they reach an al dente texture. Drain the shells and set them aside.

Prepare the Filling: In a large mixing bowl, combine the thawed and drained spinach, ricotta cheese, 1 1/2 cups of mozzarella cheese, Parmesan cheese, egg, basil, salt, and garlic powder. Stir well until fully mixed.

Preheat the Oven: Preheat your oven to 375°F (190°C).

Stuff the Shells: Spread 1 cup of marinara sauce evenly across the bottom of a 9x13-inch baking dish. Fill each cooked shell with approximately 1 to 2 tablespoons of the spinach-ricotta filling, then arrange the stuffed shells in the baking dish with the open side facing up.

Top with Sauce and Cheese: Once all the shells are filled, pour the remaining marinara sauce evenly over the top of the shells. Sprinkle the remaining 1/2 cup of mozzarella cheese over the marinara sauce.

Bake: Shield the baking dish with aluminum foil and place it in the oven for 25 minutes. Afterward, take off the foil and continue baking for another 10 minutes, or until the cheese is melted and bubbly.

Serve: Remove from the oven and let the dish cool for a few minutes before serving. Garnish with extra basil if desired.