Melt butter in a skillet over medium-low heat.
Cook apples, sugar, and water, stirring occasionally, until apples are soft and sauce has caramelized, about 20 minutes.
Top bread with heated pork, onions, and the apples. Sprinkle with watercress, and drizzle with caramelized sauce from apples and the heated reserved jus.
Delicious, but VERY sweet. I would use a little less of the Caramelized Apple sauce next time.