While pork is cooking, prepare the pickled onions. The vinegar in the onions adds a nice contrast to the pork and provides a little crunch without being overpowering. Slice the onions and place in a glass bowl or jar (no metal). Combine the apple cider vinegar, warm water, sugar, black peppercorns and kosher salt in a bowl and stir to combine until the sugar and salt are dissolved. Pour over the onions and store at room temp for at least an hour If you have leftovers, store them in the fridge, they make a great topping for a salad or sandwich as well. I used a large mason jar to pickle them and then to store in the fridge.