Blanch 2 sliced red onions in a small saucepan of boiling water to cover for 1 minute and drain in a colander. Then return them to the pan and add a cup of cider vinegar, a teaspoon of salt, and just enough cold water to barely cover the onions. Bring to a boil over high heat and simmer the onions 1 minute. Transfer the onions and brine to a glass jar and chill.
Slice leftover pork into 1/4-inch thick slices. Place 2 or 3 pieces on the roll, top with pickled onions, a slice of cheese, and some lettuce.