Steam the prepared leeks and carrots for 4-5 minutes over a pan of simmering water. Add the sugar snap peas and steam for a further 2 minutes, then add the peas for the final minute of cooking. Drain well and tip into a warmed serving dish.
Whisk all the dressing ingredients in a bowl with 1 tablespoon water. Season, pour the dressing over the warm vegetables and serve straight away.