Boil the jumbo pasta shells in a large pot of salted water until al dente, following the package instructions. Drain the shells well and rinse them under cool water to halt the cooking process.
Create the filling by combining the ricotta, 1/2 cup of mozzarella, Parmesan, egg, and minced garlic in a bowl. It is crucial to thoroughly dry the spinach by squeezing out all excess water before adding it to the cheese mixture. Stir in the Italian seasoning, salt, and pepper until just combined.
Begin preheating your oven to 375°F (190°C). Spread half of the marinara sauce in the bottom of a 9x13 inch baking dish. Use a spoon to carefully fill each cooked shell with the ricotta mixture and arrange them in the prepared dish.
Pour the remaining marinara sauce over the shells and sprinkle the remaining 1/2 cup of mozzarella cheese on top. Cover the dish with foil and bake for 20 minutes. Remove the foil and continue baking for another 10-15 minutes until the sauce is bubbling and the cheese is golden.
