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Meanwhile, bring a pan of salted water to the boil and add the new potatoes. Cook for 8-10 mins, until a knife can slide in without resistance, adding the peas for the last 2 mins. Drain, then tip back into the pan to steam dry, add the butter and malt vinegar and roughly mash, keeping it a bit chunky.
Dollop the mash onto a plate, top with the fish and serve with tartare sauce, if you like.