1. Preheat oven to 375ºF. Mist a 9-by-13-inch baking dish with cooking spray. Cook pasta shells according to package label directions; drain and set aside to cool. While pasta cooks, in a large bowl, stir together ricotta, 1 cup mozzarella, Parmesan, spinach, egg, salt, pepper, Italian seasoning and nutmeg.
2. Spread 3/4 cup spaghetti sauce over bottom of baking dish. Stuff shells with cheese-and-spinach mixture and place in dish. Spoon remaining sauce over shells and sprinkle with remaining 1 cup mozzarella.
3. Cover baking dish with foil and bake for 35 minutes. Remove foil and bake about 10 minutes longer, until bubbly and cheese begins to brown.