Bring a large pot of salted water to a boil. Add the jumbo pasta shells and cook according to package instructions until al dente. Drain and set aside.

In a large bowl, combine the ricotta cheese, half of the mozzarella, Parmesan, egg, Italian seasoning, spinach, salt, and pepper. Mix everything thoroughly.

Take each cooked shell and fill it with the cheesy mixture.

Spread a thin layer of marinara sauce on the bottom of a baking dish. Place the stuffed shells upright in the dish.

Pour the remaining marinara sauce over the shells, sprinkle the leftover mozzarella on top, and cover with aluminum foil.

Bake at 375°F (190°C) for 25 minutes. Then remove the foil and bake for an additional 10 minutes, until the cheese is bubbly and golden.

Let the stuffed shells sit for a few minutes before serving. Garnish with parsley or red pepper flakes if desired.