Preheat the oven to 200°C, then grease ovenproof casserole dish

Cut the fish into large pieces and put the pan, then poor the milk evenly over the top, bring slowly to the boil, cover and remove from heat coach for 10 minutes

Melt 30 g of butter in a frying pan and cook the onion for about 7 or 8 minutes until soft, but not brown, add the garlic and cook for another minute stop

Lift the fish from the New York and set aside. Reserve the milk stop great the lemon rind and squeeze the lemon

Add flour to the onions and cook and stir for about 1 minute stop take off the heat, and reserve the milk and whisk until smooth. Return to the heat and cook for 3 minutes. Gradually add the cream, and bring to the boil and cook for 1 to 2 minutes

Remove from the heat, allow to cool slightly, and stir in mayonnaise, gherkin, herbs, lemon rind and juice. Mix thoroughly

Arrange the fish in the prepared ovenproof dish and cover with the sauce

Boil the potatoes in a saucepan in lightly salted water for 10 to 12 minutes, until almost tender stop, drain well and grate. Melt remaining butter and mix into the potatoes. Season to taste.

Spoon the potatoes over the fish stop at this point you could put the dish into the refrigerator for up to about 8 hours, you can prepare it in the morning ready for the evening.

Bake in the preheated oven for 35 minutes, if you stored in the refrigerator add 10 to 15 minutes of cooking time.

At this point I place under the grill to lightly brown the potatoes but this really is a matter of choice.