1. Boil the potatoes in salted water until soft. Drain and mash with the butter.
2. Poach the fish in slightly salted water with parsley stalks and a slice of lemon until just cooked. Discard the parsley. Pat-dry the fish with a paper towel and flake roughly with a fork.
3. Combine the mashed potato, milk, fish, onion, zest of ½ lemon and 2 tbsp chopped parsley in a large bowl. Season to taste with salt & pepper. Form the mixture into 8 flat cakes. Lightly flour the fishcakes.
4. Beat the egg and dip each fish cake in, then coat in the breadcrumbs.
5. Heat the oil in a large fry-pan, add butter and fry the fish cakes over medium high heat for 2-3 minutes each side or until golden. Drain on paper towel.
6. Serve with tartar sauce and lemon wedges.