Ideally, use a floury, not a waxy potato variety for this recipe: the latter give the soup a slightly gluey consistency, though if you have only new potatoes to hand, be careful not to over-puree them in step 7. Maris piper, desiree, rooster or king edwards are all good bets. If you’re using multiple potatoes, rather than big baking ones, make sure they’re all roughly the same size.
Baking the potatoes will give your soup a more emphatic potato flavour. Heat the oven to 200C (180C fan)/390F/gas 5. Prick the potatoes several times with a skewer, to prevent them exploding in the oven, then put them on a lightly greased baking tray and bake for about 75 minutes, until cooked through.
Leave the potatoes to cool slightly. Meanwhile, wash the leeks well, being careful to remove any grit lurking between the layers, trim off the woody bases, then roughly chop – use the entire leek, save for the toughest top green leaves on each stalk, which you can discard.
Melt the butter in a large saucepan over a fairly gentle heat, then fry the leeks, along with a pinch of salt, until they’re soft and silky, stirring regularly so they don’t begin to stick to the pan. Scoop out a couple of spoonfuls to use as a garnish, and set aside.
Once they’re cool enough to handle, chop the potatoes into roughly evenly sized cubes, skin and all, then add them to the pan with the leeks. Saute the two together for a couple of minutes, until the potatoes are coated with butter; add a little extra fat if the mix seems on the dry side.
Add the stock or water – I like chicken stock, because it’s deliciously savoury yet more neutral than most vegetable stocks, which can be aggressively herbaceous. But use whichever you prefer, or even plain old water – though in that case you’ll need to season it well afterwards.
Bring the liquid to just below a boil, then turn the heat down and simmer gently for about 10 minutes. Leave it to cool slightly, transfer to a blender (or use a stick blender), blitz smooth, then return to the pan (if necessary).
Thin the soup with a splash of water or milk if you prefer it more liquid, then taste and adjust the seasoning. Stir in the reserved leeks, reheat and divide between bowls. Top with a spoonful of soured cream and a sprinkling of chopped chives.