Put potatoes into a deep saucepan and cover with plenty of cold salted water and boil. When they’re almost cooked, add peas and leek to the pot, bring back to the boil and cook a further 4 minutes. Test a pea — if it’s ready, tip all of the contents into a colander and run cold water over them for a minute.
While vegetables are still warm, toss with dressing and leave to cool.
This salad can be made the day before, but you will find the peas lose their vivid colour. To avoid this, cook them separately and mix in an hour or so before you serve the dish.
The easiest way to make dressing is to place oils and mint into a blender or small food processor and blend for 30 seconds, add all other ingredients and blend for a further 10 seconds. Alternatively, finely shred mint and mix it with oils. Combine lemon juice and mustard, then mix everything together.