Cook pasta according to package directions; drain and rinse under cold water until completely cool. Drain thoroughly.
Preheat oven to 350°F. Squeeze as much water as possible from the thawed spinach. In large bowl, stir together the spinach, mozzarella, ricotta, carrots, egg whites, pepper and nutmeg until thoroughly blended.
Line the bottom of an 11x17-inch baking dish with 1/2 cup tomato sauce. Divide the ricotta mixture among the pasta shells, pressing the filling into the shells with spoon to completely fill each shell. Arrange the shells side by side in a baking dish. Coat the shells with an even layer of the remaining tomato sauce. Sprinkle with Parmesan cheese, if desired.
Bake until the centres of the shells are heated through and the sauce is bubbling, about 40 minutes. Let stand 10 minutes before serving.