Bring a large pot of salted water to a boil, and then cook the jumbo pasta shells according to the package instructions until they are al dente. Once cooked, drain the shells and rinse them under cold water to stop the cooking process and prevent sticking.
In a large mixing bowl, combine the ricotta cheese, half of the shredded mozzarella, grated Parmesan cheese, chopped spinach, egg, minced garlic, dried oregano, salt, black pepper, and red pepper flakes (if using). Stir everything together until the mixture is well blended and creamy.
Pour a thin layer of marinara sauce onto the bottom of a 9x13 inch baking dish, ensuring an even coating.
Carefully stuff each cooked pasta shell with the creamy spinach filling, using a spoon to pack it in generously. Arrange the filled shells neatly in a single layer on top of the marinara sauce in the baking dish.
Drizzle the remaining marinara sauce over the stuffed shells, making sure to cover them well. Sprinkle the remaining shredded mozzarella cheese evenly over the top for a cheesy finish.
Cover the baking dish tightly with aluminum foil to keep the moisture in, and place it in the preheated oven. Bake for 25 minutes.
After 25 minutes, carefully remove the foil and return the dish to the oven. Continue baking for an additional 10-15 minutes, or until the cheese is bubbling and has turned golden brown.
Once baked, take the dish out of the oven and allow it to cool for about 5 minutes before serving.
For a fresh touch, garnish with a few fresh basil leaves just before serving, adding vibrant color and flavor to the dish.