If using frozen spinach, squeeze as much water as possible from the thawed spinach. In large bowl, mix together the spinach, mozzarella, ricotta cheese, egg whites, pepper and nutmeg. Line the bottom of a baking dish with some of the pasta sauce. Stuff each pasta shell with the cheese mixture. Arrange the shells side-by-side in the baking dish. Coat the shells with an even layer of the remaining pasta sauce. Sprinkle with the Parmesan cheese.
