Begin by bringing a large pot of salted water to a boil. Once boiling, add the jumbo shells and cook according to package instructions until al dente. Drain and set aside to cool slightly.
In a large mixing bowl, combine the thawed spinach (make sure to drain excess water), ricotta cheese, parmesan cheese, egg, salt, pepper, Italian seasoning, and garlic powder. Mix until well combined and creamy.
Preheat your oven to 375°F (190°C). Spread a thin layer of marinara sauce on the bottom of your baking dish. Using a spoon, carefully fill each jumbo shell with the spinach and ricotta mixture. Place the stuffed shells in the baking dish, open side up.
Once all the shells are stuffed, pour the remaining marinara sauce over the top, ensuring each shell is well-coated. Sprinkle the shredded mozzarella cheese generously over the sauce.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. After 25 minutes, remove the foil and bake for an additional 15 minutes, or until the cheese is bubbly and lightly golden.
Once baked, remove the shells from the oven and let them sit for a few minutes. Garnish with fresh basil or chopped parsley if desired. Serve warm and enjoy the cheesy, savory goodness of your Spinach and Ricotta Stuffed Shells!