Cook the jumbo pasta shells according to package instructions until al dente. Drain and set aside to cool.
In a large mixing bowl, combine ricotta cheese, chopped spinach, half of the mozzarella cheese, grated Parmesan cheese, egg, minced garlic, oregano, salt, and black pepper. Mix until well combined.
Preheat the oven to 375°F (190°C). Take each cooked shell and fill it generously with the spinach and ricotta mixture using a spoon or piping bag.
In a 9x13 inch baking dish, spread a thin layer of marinara sauce on the bottom. Place the stuffed shells seam side up in the dish. Pour the remaining marinara sauce over the shells and sprinkle the remaining mozzarella cheese on top.
Cover the dish with aluminum foil and bake for 25 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Allow the dish to cool for a few minutes before garnishing with fresh basil leaves. Serve hot.
Notes
Allow to cool slightly before serving for better slicing.