Curried carrot and coconut milk soup

3/4 cup finely chopped scallion
1 small onion, chopped
1 tbsp finely grated peeled fresh gingerroot
2 tbsp unsalted butter
1 tbsp curry powder
1 1/2 pounds carrots, peeled and sliced thin
2 1/2 cups low-salt chicken broth
1 to 1 1/2 cups canned unsweetened coconut milk
1 tbsp fresh lime juice plus additional to taste
Ice water for thinning soup
Trimmed scallions for garnish

1. 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.
2. In a blender puree mixture in batches with coconut milk until very smooth, transferring as pureed to a bowl. Stir in 1 tablespoon lime juice and chill soup at least 6 hour or overnight.
3. Thin soup with ice water and season with additional lime juice and salt and pepper.
Garnish soup with trimmed scallions.