Cook the jumbo pasta shells in a large pot of salted boiling water according to package instructions until al dente. Drain and rinse under cold water to prevent sticking.
In a large mixing bowl, combine ricotta cheese, half of the mozzarella cheese, Parmesan cheese, chopped spinach, egg, minced garlic, dried oregano, salt, black pepper, and red pepper flakes if using. Mix until well combined.
Spread a thin layer of marinara sauce on the bottom of a 9x13 inch baking dish.
Stuff each cooked pasta shell with the creamy spinach mixture, using a spoon to pack it in. Arrange the stuffed shells on top of the marinara sauce in the baking dish.
Pour the remaining marinara sauce over the stuffed shells and sprinkle the rest of the mozzarella cheese on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
Once done, remove from the oven and let it cool for a few minutes before serving.
Garnish with fresh basil before serving for a pop of color and flavor.
Notes
Feel free to add more spices or herbs to the filling for extra flavor.