Fillet the fish, leaving the skin on. Nail the fillet, skin side down (use only bright nails, not galvanized) to a 1-inch-thick board. Cut up 12, 1-inch sections of salt pork and enough similar-sized onions to accompany each piece of pork. On a nail, skewer a section of salt pork and a bit of onion. Do this with all the salt pork and onions. Lightly nail the salt pork-onion kebabs to the bluefish, being careful to distribute them evenly.
Prop the board straight up next to a thick bed of hardwood coals. Don’t place the fish so close to the coals that it begins to cook immediately; the idea is to slow cook, so that the oil drips out of the fish and is replaced with the onion/salt pork mixture. When the flesh is yellow but not golden brown, pull off a bit and sample. Cooking time is usually two hours or more, depending on the size of the fillet. Planked bluefish can be the crowning achievement of a shore picnic. Once you try it, you will certainly agree that it was more than worth the effort.