Trim the outer leaves from the leeks and cut off the top down to where the leaves split. Cut the leeks into 3-4 chunks, then wash really well to remove any grit. Heat a steamer, add the leeks and cook for 8 mins. Tip in the peas and cook for 2-3 mins more until both are tender.
Meanwhile, whisk the oil, vinegar, honey, mustard and seasoning together in a serving bowl to make the vinaigrette. Add the hot vegetables and herbs, and mix well.