Thai-Spiced Watermelon Soup With Crabmeat

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, about 2 hours if serving cold, or reheat in cleaned saucepan.