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Heat oven to 200C/180C fan/gas 6. Melt the butter in a saucepan, then stir in the flour for 1 min. Gradually stir in the milk and horseradish, if using. Bubble the sauce to thicken for a couple of mins, then add the dill, three-quarters of the onions, the lemon zest, fish and some seasoning. Pour into 4 individual pie dishes or 1 large pie dish.
Mix the mashed potato with the remaining spring onion. Spoon over the fish mixture, then bake for 15-20 mins until golden and bubbling.