Peel the potatoes and chop into small chunks. Cook in boiling water for approximately 10 mins until tender. Drain, then add butter, season and mash roughly.
Meanwhile in a large deep frying pan, gently cook the fish in the milk (in 2 batches if necessary) for 8 mins or until it flakes. Lift fish out of milk and set aside on a plate. Chop watercress.
When fish is cool enough to handle, flake it into a large bowl, add mashed potato, watercress and spring onions and grate in the lemon zest.
Gently combine all the ingredients then shape into 8 fishcakes, using your hands. Leave to chill in the fridge for ½ hour.
Heat 2tsp oil in a frying pan, dust both sides of the fishcakes with flour and gently fry on each side until they are browned and hot to the centre. Serve with watercress salad.
Recipe by appetite magazine at https://www.appetitemag.co.uk/smoked-haddock-and-watercress-fishcakes/