Peel, core and 1/4 apples; place in 2 quart baking dish. Melt butter in medium saucepan; blend in flour. Add sugar and water, stirring over low heat until dissolved. Add marshmallows and continue stirring until melted. Bring sauce to boil; pour over apples. Bake at 350~ for 25 to 30 minutes, basting with sauce occasionally. Serve warm or cold.