Bring cider to a boil in a large saucepan; cook until reduced to 1/2 cup (about 15 minutes). Remove from heat. Add butter; stir with a whisk until butter melts.
Place 2 bread halves on each of 8 dessert plates. Divide cheese evenly among bread halves. Place 2 apple slices on each bread half. Drizzle 1 tablespoon cider reduction around each serving.