Cold Curried Carrot and Coconut Milk Soup

3/4 cup finely chopped scallion (about 1 bunch)
1 small onion, chopped (about 2/3 cup)
1 tablespoon finely grated peeled fresh gingerroot
2 tablespoons unsalted butter
1 tablespoon curry powder
1 1/2 pounds carrots, peeled and sliced thin (about 4 cups)
2 1/2 cups low-salt chicken broth (or vegetable stock or water)
1 to 1 1/2 cups canned unsweetened coconut milk
1 tablespoon fresh lime juice plus additional to taste
ice water for thinning soup (optional)
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 broth. Simmer mixture, covered, 20 minutes, or until carrots are very soft.

In a blender 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 and season with additional lime juice and salt and pepper. Garnish soup with trimmed scallions.