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Mix together 125ml mayonnaise, 1 rounded tbsp roughly chopped capers, 1 rounded tsp creamed horseradish, 1 rounded tsp Dijon mustard, 1 small very finley chopped shallot and 1 tsp finely chopped flatleaf parsley. Set aside.
Lay 450g skinned Icelandic cod or haddock fillet and 2 bay leaves in a frying pan. Pour over 150ml milk and 150ml water.
Cover, bring to a boil, then lower the heat and simmer for 4 mins. Take off the heat and let stand, covered, for 10 mins to gently finish cooking the fish.
Lift the fish out of the milk with a slotted spoon and put on a plate to cool. Drain the potatoes in a colander and leave for a min or two.
Tip the potatoes back into the hot pan on the lowest heat you can and let them dry out for 1 min, mashing them with a fork and stirring so they don't stick. You should have a light, dry fluffy mash.
Take off the heat and beat in 1 rounded tbsp of the sauce, then ½ tsp lemon zest, 1 tbsp chopped flatleaf parsley and 1 tbsp snipped chives.
Season well with salt and pepper. The potato should have a good flavour, so taste and adjust to suit.
Drain off liquid from the fish, grind some pepper over it, then flake it into big chunks into the pan of potatoes.
Using your hands, gently lift the fish and potatoes together so they just mix. You'll only need a couple of turns, or the fish will break up too much. Put to one side and cool.
On the floured board, and with floured hands, carefully shape into four cakes, about 2.5cm thick. One by one, sit each cake in the egg, and brush over the top and sides so it is completely coated.
Sit the cakes on the crumbs, patting the crumbs on the sides and tops so they are lightly covered. Transfer to a plate, cover and chill for 30 mins (or up to a day ahead).
Heat 3-4 tbsp vegetable or sunflower oil in a large frying pan. To test when ready, drop a piece of the dry breadcrumbs in - if it sizzles and quickly turns golden brown, it is ready to use.
Fry the fish cakes over a medium heat for about 5 mins on each side or until crisp and golden. Serve with the rest of the sauce (squeeze in a little lemon to taste), lemon wedges for squeezing over and watercress.