Heat the oil in a saucepan, add the leeks, toss in the oil then cover and fry gently for 10 minutes, stirring occasionally, until softened but not coloured. Mix in the peas and cook briefly.
Pour the stock into the pan, add a little salt and pepper then bring to the boil. Cover and simmer gently for 10 minutes.
Ladle half the soup into a blender or food processor, add all the mint and blend until smooth. Pour the purée back into the saucepan. Mix the mascarpone with half of the lemon rind, reserving the rest for a garnish. Spoon half the mixture into the soup, then reheat, stirring until the mascarpone has melted. Taste and adjust the seasoning if needed. Ladle the soup into bowls, top with spoonfuls of the remaining mascarpone and a sprinkling of the remaining lemon rind. Garnish with mint leaves and lemon rind curls, if liked.