Melt the butter in a saucepan. Add the onion and fry until soft but not brown. Stir in the flour and cook, stirring for 1 minute. Remove from the heat and gradually stir in the stock and orange juice.
Return to the heat and bring to the boil, stirring. Simmer until thickened and smooth. Add the chives, salt and pepper, nutmeg, allspice and carrots. Cover and simmer 1 hour. Puree in a blender until smooth. Allow to cool, then chill at least 2 hours before serving.