In skillet, melt 1 tbsp. butter over medium-high. Season chicken; cook, skin-side down, until golden, about 8 minutes. Turn, add cider, then roast at 375 degrees until cooked through, 20 minutes; transfer to plate. Whisk mustard and sage into sauce. In another skillet, cook apples in remaining butter over medium-high, stirring, 5 minutes; season.