This is an old, old recipe but surprisingly good.
You will need: 1-2 onions, sliced thin; several cooking apples, peeled and sliced (depends on how many you're serving), a couple of tablespoons butter, and brown sugar to taste
Melt butter in a heavy skillet (cast iron works well, if you have it). Fry the onion slices in the butter over moderate heat until golden. Add the apple slices and cover, cook until apples are tender. You want this to cook rather slowly so the onions get nice and sweet. When the apples are done, add brown sugar to taste. If you want, you can add bacon (fried and cut up) for flavor. My family prefers it without the bacon.