• 50g butter
• 2 medium potatoes, peeled and cut into 1cm pieces
• 1 small onion, cut the same size as the potatoes
• 2 white parts of leeks, sliced (save the green tops for another soup or stock)
• 1½-2pts vegetable stock
• double cream (optional)
• 125ml full-fat milk
Melt the butter in a heavy saucepan. When it foams, add the potatoes, onion and leeks and toss them in the butter until they are well coated. Season well with salt and freshly ground pepper and toss again. Put a disc of greaseproof paper (called a cartouche by chefs) on top of the vegetables to keep in the steam), then cover the pan with its lid. Cook over a gentle heat for 10 minutes, or until the vegetables are soft but not coloured.
Uncover the pan and discard the paper. Pour in 850ml/11⁄2 pints of the 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.
Purée 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 the milk.
Drizzle the cream over each serving, top with a scattering of chives and black pepper and serve at once.
