Peel the carrot and swede and cut them both into a 1cm (½") dice.
If needed, cut the potatoes into a 2-2½cm (¾") dice
Heat a medium saucepan (20cm or 8") over a medium heat, and when it's hot, add the butter.When the butter foams, season the lamb with the salt, add it to the butter and cook for 3-4 minutes. The idea is to just get the juices flowing and flavour the butter rather than colour the lamb.
Add the onion and cook for 5 minutes to begin to soften.
Add the potatoes, swede, and carrot, then pour over the stock, stir and bring to a boil.
Skim off any foam that forms and pop on a lid and reduce the heat so that the soup sits at a gentle simmer, then cook for 60 minutes.
Cut the leek into ½-1cm (¼-½") thick coins and add them to the soup, partially cover and cook for another 20 minutes.