1 Cut watermelon and cut flesh from the shell. Cut watermelon into large cubes and remove the seeds. Puree flesh with an immersion blender.
2 Peel and finely chop the ginger, garlic and shallots.
3 Separate the lemongrass, taking care to remove the outer leaf layers. Trim off both ends and chop the soft parts of the lower third finely.
4 Heat the oil in a pot and add the shallots, ginger, garlic and lemongrass. Stir and cook over medium heat for 4-5 minutes. Add a whole chile pepper to the pot.
5 Put 1/3 of the melon puree into the pot, bring to a boil and cook for 5 minutes over medium heat.
6 Squeeze lime. Puree the soup with an immersion blender and mix with the remaining watermelon puree. Season with lime juice, salt and pepper.
7 Put the soup through a fine sieve and rub with the back of a spoon. Refrigerate for at least 3 hours.
8 Just before serving, rinse the scallions, trim and chop the whites very fine. Pick over crabmeat, discarding shell pieces. Mix crab with the scallions.
9 Rinse cilantro, shake dry, pluck off the leaves, chop and add to the crabmeat. Mix with sesame oil and season with salt, pepper and lime juice. Place the crab meat in small bowls and serve alongside the soup.