Score the fat in a 1/2-inch pattern with a sharp knife. If the belly is lean, you can skip scoring.
Make the Marinade
Add all ingredients except the pork, pineapple, olive oil, and onion to a blender. Blend until smooth.
Reserve 3 tablespoons of the marinade for the pineapple sauce and refrigerate.
Marinate the Pork
Add the pork and remaining marinade to a freezer bag or container and marinate in the refrigerator for 4 to 8 hours.
Braise the Belly
Heat oven to 325°F.
Remove the pork from the marinade and roll it tightly with the fat side facing outward. Tie at 2-inch intervals with butcher's twine. You can leave the belly flat if you do not want to roll it.
Place the chopped onion in the bottom of an ovenproof pan. Set the pork on top.
Add enough water or stock to come halfway up the pork belly. Pour any extra marinade over the pork, but not the reserved 3 tablespoons.
Cover and braise for 2 hours.
Remove the cover and braise for one more hour, or until fork-tender.
Broil
Remove the pork from the pan. Turn the oven to broil and broil the pork until browned. A little char is ok.
Remove from the oven and set aside.
Carefully remove about 1/4 cup of the onions from the braise. You will use them for the sauce.
Make the Sauce
Turn on your oven's broiler.
Place the pineapple in a bowl and mix it with the olive oil. Place on a greased baking pan.
Broil until lightly charred and tender. Let it cool slightly.
Carefully add the broiled pineapple, reserved marinade, and reserved cooked onions to a blender. Blend until smooth. Season with salt and pepper to taste.
Serve
Slice the pork and serve it with the pineapple sauce. It is especially good over coconut rice.
NOTES
Marinate 4 to 8 hours. That is enough time for the jerk marinade to flavor the pork without taking over the dish. Flip the belly halfway through if you can.
Score the fat first. Cut a shallow 1/2-inch pattern on the fat side so the seasoning and heat work more evenly. If the belly is very lean, you can skip this step.
Roll it if wanted. Rolling and tying the belly is optional, but it helps it cook more evenly and makes cleaner slices later. Tie it at about 2-inch intervals with the fat side facing out.
Keep liquid halfway up. Add enough water or stock to come about halfway up the pork belly, not over the top. This is a braise, not a boil.
Cover, then uncover. Braise covered at 325 F for 2 hours, then uncover and cook 1 hour more until the pork is fork-tender. The covered time tenderizes it, and the uncovered time helps the outside dry slightly before broiling.
Watch the broiler closely. Broil just until browned and lightly charred. Pork belly can go from browned to burnt fast, especially with the marinade sugars and rendered fat.
Reserve marinade safely. Set aside the 3 tablespoons for the pineapple sauce before it touches the raw pork. Then blend it with the broiled pineapple and some cooked onions from the braise.
Make ahead if needed. The marinade can be made the day before, and the pineapple can be broiled ahead and reheated before blending the sauce. Leftovers keep in the refrigerator for up to 3 days.