Put the potatoes into a pan of cold water with a teaspoon of salt, bring to a simmer and cook for 15 minutes.
Meanwhile, in a large pan, add the cold water, milk and onion and bring to a simmer. Add the fish and cook for 5 minutes.
Remove the fish with a slotted spoon onto a clean plate, cover with cling film and keep warm. Add the leeks and cabbage to the cooking liquid and cook for a further 7 minutes.
Drain the potatoes and mash until smooth.
Stir in enough mash to the leeks and cabbage mixture to thicken the broth. Season to taste - you may need to add a little pepper but probably no salt.