Wash and trim spinach, leaving the water that clings to the leaves. Place in a large skillet and steam just until wilted, about 3-5 minutes. Drain and chop; set aside. Melt butter in a small saucepan over medium heat. Blend in flour and pepper; cook and stir until bubbly. Slowly add milk; bring to a boil, stirring constantly. Cook and stir 1 minute. Remove from the heat. Stir in onion, salt and nutmeg. In a large bowl, beat egg whites until soft peaks form. In another bowl, beat egg yolks until thick and lemon-colored; stir into white sauce. Gently fold into egg whites along with spinach. Pour into a greased 1-1/2-qt. casserole or souffle dish. Place casserole in a larger pan; fill larger pan with 1 in. of water. Bake, uncovered, at 350° for 50-60 minutes or until a knife inserted halfway between the center and edge comes out clean. Serve immediately.