1. In small bowl or tightly covered jar, combine oil, vinegar, chives, 1 teaspoon salt, the sugar, pepper, and mustard. Stir or shake until well blended. Refrigerate until needed.
2. Trim leeks cut off root ends and green stems. Cut each in half lengthwise. Wash thoroughly.
3. In 4-quart kettle, bring 2 quarts water to boil, add leeks and salt, simmer, covered until tender. Drain leeks and cool 30 minutes.
4. Arrange leeks in shallow dish, shake dressing well and pour over leeks. Refrigerate, covered, until well chilled.
5. Remove leeks from dressing with slotted utensil, and arrange in center of chilled serving dish.
6. Place endives, at each end, and drizzle with dressing.
7. Garnish with water cress and serve cool.