Cut chicken into small chunks, then toss in the cornflour. Heat the oil in a large pan, add the leeks, then fry for 3 mins until starting to soften. Add the chicken and 7 oz of water from the potatoes, then bring to the boil, stirring. Reduce the heat, then gently simmer for 10 mins, until the chicken is just tender. Remove from the heat, then stir in the lemon zest, parsley, crème fraîche and mustard.