Trim ragged green ends of leeks and roots. Slice leeks in half lengthwise and in half again. Wash carefully under cold running water. Place leeks on their sides in a lasagna pan large enough to hold them and add 1/4 inch water. Microwave on high, uncovered, for 10 minutes, or until leeks are tender. Or boil leeks in a skillet large enough to fit them lying on their sides and covered with water.

While leeks are cooking, make vinaigrette. Whisk lemon juice and mustard together until smooth. Add chicken stock and oil. Add salt and pepper to taste. When leeks are cooked, carefully lift them out of water and drain on a rack in the sink. Place leeks on an oval dish and spoon vinaigrette over the top.