In a bowl, combine brown sugar, flour and nutmeg; cut in butter until the mixture resembles coarse crumbs. Stir in pecans. Place sweet potatoes and peaches in a shallow 2-qt. broiler-proof baking dish. Sprinkle with pecan mixture.
Bake, uncovered, at 350° for 35 minutes. Sprinkle with marshmallows. Broil 4-6 in. from the heat until marshmallows are golden brown.