Preheat oven to 325°F.
Place butter, salt, mustard, ginger and pepper in a medium bowl. Stir together to form a paste.
Rub butter paste over surfaces of chops.
Place a heavy, over-proof frying pan over medium heat.
When hot, brown chops well on both sides. Remove from frying pan.
Add onion to frying pan. Sauté until well-browned.
Return chops to pan. Top with the onions.
Add rum.
Cover tightly. Place in preheated oven and until meat is very tender, about 1 1/2 hours. If necessary, add more rum to keep moist.
Remove from oven.
Put chops on warm serving platter or plates.
Add pineapple to juices and onions remaining in the pan. Stir to mix. Place back in oven for about 3 - 5 minutes just to heat through.
Spoon pineapple/onion mixture over the chops.
Garnish with watercress.
Serve hot.