Peel and chop the potatoes into even-sized chunks. Place into a saucepan, cover with water and boil for 15 minutes or until a fork can easily press through the potato.
Drain the water off the potatoes, mash with a fork and set aside.
Lay the pieces of fish in a large frying pan. Pour over the milk and water. Cover with a lid and bring to a boil. Lower the heat to a simmer and continue simmering for 5 minutes. Remove the pan from the heat and set aside (covered) for 10 minutes (the fish will continue to cook during this time). Gently remove the cooked fish and break into large pieces.
In a large bowl, place the mashed potato, fish pieces, egg, butter, chives, lemon zest, sour cream, salt and pepper. Mix together with a spoon until completely combined and the fish has flaked into smaller pieces.
Place the mixture into the fridge for 15 minutes.
Form the mixture into 8 equal sized patties. Flatten them slightly with your hands.
Coat the fish patties in the panko crumbs and place onto a plate. Chill in the fridge for a further 15 minutes (minimum). You can chill them for up to 1 day ahead if you like.
Heat 3-4 tablespoons of vegetable oil in a frying pan over medium heat. In batches, cook the fish cakes for 5 minutes on either side or until golden and crisp.
Serve with tartare sauce and salad.