This light, spicy soup is delicious hot or chilled. The sautéed aromatics turn the broth a rich red-orange, more reminiscent of tomato than watermelon. See if your guests can guess the main ingredient.
Active time: 1 hr Start to finish: 1 hr (3 hr if serving chilled)
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.
03Cook 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.
04Remove 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).
05Add 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.
06Toss crabmeat with cilantro, oil, and salt.
07Divide crab among 4 soup plates, mounding in center, and pour chilled or hot soup around it.