Preheat the oven to 375°F (190°C).
Cook the pasta shells according to package instructions until al dente.
In a large bowl, combine chopped spinach, ricotta cheese, mozzarella cheese, Parmesan cheese, beaten egg, Italian seasoning, and salt and pepper.
Carefully stuff each pasta shell with the cheese and spinach mixture.
Spread half of the marinara sauce on the bottom of a baking dish.
Place the stuffed shells in the baking dish and pour the remaining marinara sauce over the top.
Sprinkle any remaining mozzarella and Parmesan cheese on top.
Cover the baking dish with foil and bake for 25 minutes.
Remove the foil and bake for an additional 10-15 minutes until the cheese is bubbly and golden.