Spread the sliced onions across the bottom of a 4- to 6-quart slow cooker.
Arrange the pork chops over the onions in one layer, overlapping slightly if needed.
Whisk the brown sugar and apple cider vinegar together until the sugar mostly dissolves. Pour the mixture over the chops and onions.
Cover and cook on Low for 6 to 7 hours or High for 3 to 4 hours, until the chops are very tender. Keep the lid closed during cooking.
For a thicker glaze, uncover and let the sauce cook on Low for another 15 to 20 minutes.
Serve the pork chops with the soft onions and spoon the sweet and sour cooking juices over the top.