Heat 2 tablespoons butter in a saucepan. Cook onions, covered until soft. Stir in curry powder. Add carrots and broth; bring to a simmer and cook for 20 minutes or until carrots are very tender. Season to taste with salt and pepper.
Puree in food processor or blender with orange rind and juice.
Return soup to saucepan (boil down if too thin; add water if too thick); adjust seasoning. Portion soup into bowls and garnish with a dab of sour cream and a sprinkling of scallion.