Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9×13 inch baking dish.
Step 2: Cook the jumbo pasta shells according to package directions for al dente. Drain, rinse with cold water, and set aside to prevent sticking.
Step 3: Ensure the spinach is completely thawed (if frozen) or wilted (if fresh) and thoroughly squeezed dry to remove all excess moisture. Chop finely.
Step 4: In a large bowl, combine the drained ricotta cheese, chopped spinach, beaten egg, 1/2 cup Parmesan cheese, 1 cup mozzarella cheese, garlic powder, dried oregano, ground nutmeg, salt, and black pepper. Mix until well combined.
Step 5: Spread about 1 cup of marinara sauce on the bottom of the prepared baking dish.
Step 6: Carefully stuff each cooked pasta shell with the ricotta-spinach mixture using a small spoon or piping bag. Arrange the stuffed shells in a single layer in the baking dish.
Step 7: Pour the remaining marinmarinara sauce over the stuffed shells, ensuring they are mostly covered. Sprinkle generously with the remaining 1 cup of shredded mozzarella cheese.
Step 8: Cover the baking dish tightly with aluminum foil. Bake for 25-30 minutes, or until the sauce is bubbling and the shells are heated through.
Step 9: Remove the foil and bake for another 10-15 minutes, or until the mozzarella cheese is melted, bubbly, and lightly golden brown.
Step 10: Let the dish rest for 5-10 minutes before serving. Garnish with fresh parsley or basil, and serve hot.