Lay the fish in a large frying pan with bay leaves, milk and water. Cover and bring to the boil, then lower the heat and simmer for 4 minutes. Remove from the heat and let stand, covered, for 10 minutes to gently finish cooking the fish.
Meanwhile, place the potatoes into a pot of boiling salted water and simmer until just tender. Drain the potatoes in a colander, allowing them to stand in the colander for a few minutes to dry out. Then return the potatoes to the pot and mash to a fluffy consistency.
Lift the fish out of the milk with a slotted spoon and set aside on a plate to cool slightly.
Transfer the potatoes to a large mixing bowl and add 2 tbsp of the tartare sauce, the lemon zest, spring onions and parsley. Season with salt and pepper to taste.
Drain off any excess liquid from the fish and, using your hands, roughly flake it into the potato mix. Continue to gently mix using your hands until the fish and potatoes are roughly combined. Set aside to cool to room temperature.
Prepare a crumbing station using 3 bowls – one each for the flour, beaten egg and breadcrumbs.
Divide the fish mixture into 8 portions and carefully shape into cakes about 3cm thick. One at a time, roll in the flour then dip into the egg to coat. Finally, place into the breadcrumbs and press gently to coat on all sides. Transfer to a plate, cover and refrigerate for 30 minutes.
Heat the oil in a large non-stick frying pan over medium heat. Fry the fish cakes for 5 minutes on each side, or until golden and crispy. Serve immediately with the remaining tartare sauce, lemon wedges and a leafy green salad.