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Heat oven to 200C/180C fan/gas 6. Cook the potatoes and onion in boiling, salted water for 6-7 mins until almost tender. Drain well. Grease a 20 x 20cm baking dish with a little of the butter, then arrange the potatoes in it.
Break the fish into chunky pieces, discarding any skin and bones. Tuck the pieces in and around the potatoes, then scatter over the dill. Whisk the cream, stock and horseradish together and season. Pour over the potatoes. Dot the remaining butter over the top, then bake for 20-25 mins until golden. Serve with a green salad, if you like.