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Put the leeks, potato and stock in a pan and bring to the boil. Cover and simmer for 8 mins.
Tip in the peas, cover and cook for 5 mins more. Take off the heat and blitz with a hand blender (or in a food processor) until smooth, then stir in the yogurt and mint.
Meanwhile, lay the slices of prosciutto in a large non-stick frying pan in a single layer and heat until crisp. Allow to cool a little, then tear into strips, ready to sprinkle over the soup with some ground black pepper. Will keep in the fridge for 2 days.