Preheat the oven to 375°F (190°C).
Cook the jumbo shells according to package directions until just al dente. Drain and set aside to cool slightly.
Thaw the spinach and drain well. Wring it in a clean towel until as much moisture as possible is removed.
Thinly slice the basil. Shred the mozzarella and grate the Parmesan, keeping them separate.
In a large mixing bowl, stir together the spinach, ricotta, 1 cup of the mozzarella, 1/2 cup of the Parmesan, the egg, basil, salt, and pepper until thoroughly combined.
Pour 2 cups of the marinara sauce into the bottom of a 13x9-inch baking dish.
Stuff each pasta shell with a generous amount of the spinach and ricotta mixture. Place the stuffed shells in the baking dish.
Cover with the remaining sauce and top with the remaining mozzarella and Parmesan.
Cover with aluminum foil and bake for 25 minutes.
Remove the foil and continue baking for 10 to 15 minutes, or until the top begins to brown and the sauce begins to bubble.
