Discard 1 or 2 outer leaves of lemon grass and trim root ends. Finely chop enough lemon grass from lower 6 inches of stalks to measure 1/4 cup total.
In a small saucepan combine lemon grass and 1 cup tomato water and simmer uncovered, 10 minutes. Pour mixture through a fine sieve into another small saucepan, discarding solids, and stir in gelatin. Heat mixture over low heat, stirring until gelatin is dissolved. In a large bowl stir gelatin mixture into remaining 5 cups tomato water. Chill tomato-water mixture, covered, at least 8 hours and up to 1 day. Discard rind and seeds from watermelon and finely chop enough fruit to measure 3/4 cup.
Divide soup among 6 bowls and just before serving add watermelon, chives, and mint
Rinse tomatoes well under running water and quarter. In a food processor puree tomatoes with salt until smooth. Line a large sieve set over a tall nonreactive kettle with cheesecloth and carefully pour tomato puree into center of cheesecloth. Gather sides of cheesecloth up over puree to form a large sack and, without squeezing puree, gently gather together upper thirds of cheesecloth to form a neck. Carefully tie neck securely with kitchen string. Tie sack to a wooden spoon longer than diameter of kettle and remove sieve. Put spoon across top of kettle, suspending sack inside kettle and leaving enough room underneath sack so that it will not sit in tomato water that accumulates. Let sack hang in refrigerator at least 8 hours.
Without squeezing sack, discard it and its contents and transfer tomato water to a bowl. Tomato water keeps, covered and chilled, 4 days. Yield: about 6 cups
*available at Southeast Asian markets and some specialty foods shops.
Recipe courtesy of Gourmet Magazine