1 tablespoon fresh lime juice plus additional to taste
trimmed scallions for garnish
In a large heavy saucepan cook chopped scallion, onion, and gingerroot in butter with curry powder and salt and pepper to taste over moderately low heat until softened and add carrots and stock. Simmer mixture, covered, 20 minutes, or until carrots are very soft.
In a blender or vitamix purée mixture in batches with coconut milk until very smooth, transferring as puréed to a bowl. Stir in 1 tablespoon lime juice and chill soup at least 6 hour or overnight.
Thin soup with ice water if it is too thick, and season with additional lime juice and salt and pepper.
Garnish soup with trimmed scallions, or a dollop of yogurt and chopped cilantro.