Step 1/8
Peel and cut 450g potatoes, into 1cm cubes, 1 small onion, cut the same as the potatoes, and 450g of the WHITE parts of leeks. Melt 50g butter in a heavy saucepan. When it foams, add the potatoes, onion, and the leeks and toss them in the butter until they are well coated.
Step 2/8
Season well with salt and freshly ground pepper and toss again, then cover the pan with its lid.
Step 3/8
Cook over a gentle heat for 10 mins, or until the vegetables are soft but are not coloured.
Step 4/8
Pour in 850ml of the light chicken or vegetable stock, bring to the boil and simmer until the vegetables are just cooked – about 5 minutes. Do not overcook or the soup will lose its fresh flavour.
Step 5/8
Next, purée the vegetables in a blender until silky smooth, in batches if necessary, then taste and adjust the seasoning. Return the soup to a clean pan and stir in three quarters of a 142ml carton of whipping cream and 125ml of full-fat milk.
Step 6/8
To finish the soup, finely shred the white part of 1 leek and gently cook it in a small knob of hot butter for a few mins until it is softened but not coloured.
Step 7/8
Reheat the soup to a gentle simmer (add some extra stock at this point if the soup is too thick for your liking), then pour into warmed bowls.
Step 8/8
Drizzle the remaining cream over each serving, top with a little pile of buttered leeks and a scattering of chopped chives and black pepper and serve at once.