|Cider vinegar||3 Tablespoon|
|French mustard||1 Teaspoon|
|Onion||1 Ounce, peeled and finely chopped|
|Caster sugar||2 Teaspoon|
|Freshly ground black pepper||To Taste|
Trim about half the green part from the leeks. Cut the remainder of the leeks into 3 mm (1/8 inch) slices and wash thoroughly. Drain, then blanch in boiling salted water for 3 to 4 minutes. Drain and cool quickly in cold water. Drain well.
In a screwtop jar, shake together the remaining ingredients. Pour over the leeks and toss together. Leave to marinate for at least 2 hours.