Heat oven to 350 degrees. Beat egg whites until stiff peaks form; set aside. Beat egg yolks; blend in lemon peel, juice and milk. Add sugar, flour and salt; beat until smooth. Gently fold in egg whites. Pour into ungreased 1 quart casserole. Place casserole dish in pan of very hot water, 1-2 inches deep. Bake 45 to 50 minutes. Serve hot or warm.