Scrape and thinly slice the carrots. Peel and finely chop the onion. Melt the butter in a large saucepan over low heat, add the carrots and onion, cover with a pan lid, and cook gently for 5 minutes to soften the onion but not brown.
Pour the stock into the pan, add the sugar and season with salt and pepper. Bring to the boil, then lower the heat and cook gently, covered, for 40 minutes until carrots are soft. Pass soup mixture through a food mill, or purée in an electric blender or food processor. Stir in the orange juice. Allow to cool, then chill. Stir in the cream and finely chopped chives. Serve in chilled soup bowls.
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