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To make the mash, boil the celeriac in salted water for 10 mins until soft. Drain, put back in the pan and steam-dry for a few mins. Mash with the butter and seasoning, then stir in most of the parsley. Keep warm while you cook the fish.
Heat half the oil in a frying pan. Dust the fish in flour, season on both sides, and fry for about 4 mins on each side. Remove to a plate. Add the remaining oil to the pan and cook the garlic and chilli for 2 mins until golden. Add the vinegar, sugar and a little salt, then bubble for 1-2 mins. Return the fish to the pan to warm through.
Serve the fish on the mash, with the pan sauce poured over. Sprinkle with the remaining parsley and serve with lemon.