Cook the pasta shells according to package directions until al dente. Drain and set aside to cool slightly.
In a large mixing bowl, combine the chopped spinach, ricotta cheese, half of the mozzarella cheese, Parmesan cheese, beaten egg, garlic powder, red pepper flakes (if using), salt, and pepper. Mix until well blended.
Carefully stuff each pasta shell with the cheese and spinach mixture and place them in a greased baking dish.
Pour the marinara sauce evenly over the stuffed shells, ensuring they are all covered.
Sprinkle the remaining mozzarella cheese on top of the sauce.
Cover the dish with aluminum foil and bake in the preheated oven for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes, until the cheese is bubbly and slightly golden.
Let the dish cool for a few minutes before serving.
Notes
Garnish with fresh basil leaves and serve warm with a side salad for a delightful meal.