In a large pot, bring salted water to a rolling boil. Carefully add the jumbo pasta shells and cook according to the package instructions until al dente. Once cooked, drain the shells and set them aside to cool slightly to handle.
In a large mixing bowl, thoroughly combine the ricotta cheese, chopped spinach, half of the shredded mozzarella, grated Parmesan, beaten egg, minced garlic, dried oregano, salt, and black pepper. Mix until the ingredients are evenly incorporated and creamy.
Preheat your oven to 375°F (190°C). Using a teaspoon or a piping bag, carefully fill each cooled pasta shell with the rich ricotta-spinach mixture, pressing gently to pack the filling in.
Spread 1 cup of marinara sauce evenly over the bottom of a 9x13 inch baking dish. Arrange the stuffed shells in a single layer on top of the sauce. Pour the remaining marinara sauce over the shells, ensuring they are well coated. Sprinkle the top with the reserved mozzarella cheese.
Cover the baking dish tightly with aluminum foil and place it in the preheated oven. Bake for 25 minutes, then remove the foil and let it bake uncovered for an additional 10-15 minutes, or until the cheese is beautifully bubbly and golden.
Once baked, allow the dish to rest for a few minutes before serving. Garnish generously with fresh basil leaves for an aromatic touch and vibrant color.
Notes
For a richer flavor, add more garlic or herbs to the filling.