Preheat the oven to 375°F (190°C).
Cook the pasta shells according to package instructions until al dente. Drain and set aside.
In a bowl, mix the ricotta cheese, spinach, mozzarella, Parmesan, egg, Italian seasoning, salt, and pepper.
Stuff each pasta shell with the cheese mixture using a spoon or piping bag.
Spread a layer of marinara sauce on the bottom of a baking dish.
Place the stuffed shells in the dish, seam side up.
Cover the shells with the remaining marinara sauce.
Sprinkle additional mozzarella cheese on top, if desired.
Cover with foil and bake for 20 minutes.
Remove foil and bake for an additional 10 minutes until cheese is bubbly and golden.