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Heat oven to 180C/160C fan/gas 4. Cut the ends from the potatoes and keep. Cut the peel and 1cm flesh from the potatoes, leaving blocky barrels of potato behind. On a shallow baking tray, massage the skins and potato ends with the oil, salt and thyme sprigs. Roast for 30 mins, turning halfway through, until tender and starting to crisp. Set aside until later.
Simmer the salad onion with the vinegar until reduced to 1 tsp or so. Strain the vinegar into the butter (keep the onion), add the crab and some seasoning, and mash well. Chill until needed.
Heat grill to high – make sure it’s really hot before you cook the fish. Slash the skin of each fillet three times and season. Put the potatoes under the grill for 2 mins or until hot and starting to crisp again, then lay the fish fillets on top. (If your grill doesn’t go very high, fry your fillets, skin-side down, in a non-stick frying pan to crisp it up before putting them on top of your potatoes.) Pour olive oil over the fish, and grill for 4 mins or until opaque with the skin crisping.
Dollop ¼ of the crab butter onto each fillet, then grill for 1 min until the butter begins to melt. Using a fish slice, transfer the fish to warmed plates to serve, with the potato skins, a few thyme leaves and a charred onion & tomato salad.