Thai-spiced watermelon soup with crab meat

Make soup:
If you like substance in your soup, mash and chop – do not blend
Purée watermelon in a blender until smooth and transfer to a bowl. (Don't wash blender.)
Discard 1 or 2 outer leaves of lemongrass and trim root end.
Thinly slice lower 5 to 6 inches of stalk and then mince, discarding remainder.
Cook lemongrass, shallot, ginger, and garlic in oil in a 2-quart heavy saucepan over moderately low heat, stirring, until aromatics are pale golden, about 5 minutes.

Add about one third of watermelon purée and simmer over moderate heat, stirring, 5 minutes.
Remove watermelon mixture from heat, then transfer to blender along with chile, lime juice, and salt and blend until smooth (use caution when blending hot liquids).

Add remaining watermelon purée and blend briefly. Season soup with more chile, lime juice, and salt if desired, blending if necessary.
Pour soup through a sieve into a bowl, pressing on and then discarding any solids. Chill soup uncovered, for about 2 hours if serving cold, or reheat in cleaned saucepan.

Prepare crab:
Toss crabmeat with cilantro, oil, and salt.
Divide crab among 4 soup plates, mounding in center, and pour chilled or hot soup around it.

Cooks' notes:
• Soup can be made 1 day ahead and chilled. Cover after 2 hours of chilling.
• Crabmeat can be picked over 1 day ahead and crab mixture can be prepared 1 hour before serving. Chill, covered.
*Available at Asian markets and some supermarkets.