Place the potato in a pot, fill with enough water to cover. Bring to boil and cook for about 20 minutes. Drain and set aside to cool in a large bowl. While the potatoes are cooking, mix apple cider vinegar, Dijon mustard and mayonnaise. Add onion, apple and boiled egg. Pour this mixture into the bowl of potatoes and stir well. Serve immediately or refrigerate for a day to let the flavors blend together.