Place the fish fillets in a large saucepan, pour in the milk, add the bay leaf and season with salt and pepper. Cover the pan with a lid and gently bring milk to a low simmer. Simmer gently and poach for some 10 minutes or so or until the fish is just cooked through.
When ready, carefully remove the fish and transfer to a plate lined with kitchen paper. Remove the skin and break the fish into large chunks. Set aside and leave to cool.
Meanwhile, cook the potatoes: place in a pan of cool water, bring to the boil and cook till tender. Drain well, then return to the pan and mash with a bit of the milk that you poached the fish in. Season to taste.
Mix the fish, egg, spring onion, lemon juice and zest, dill and parsley together until well combined. Take care not to break up the fish too much. Add potatoes to thicken mixture sufficiently to roll into balls. Season to taste and roll into balls.
Roll the balls in the breadcrumbs, coating each one evenly all over. Flatten into burger shapes to size you desire. Set aside on tray, cover with clingfilm and chill in the fridge for an hour or so - or up to a maximum of 24 hours.
Heat oil in large frying pan, add fish cakes and cook on medium heat, turning to ensure golden brown on both sides.
Can save the milk used to poach fish and make a sauce, like parsley sauce.