Sauté the celery and onion in the oil for 3 minutes, then add the water and bring to the boil. Lower the heat and simmer for 10 minutes. Strain, reserving the liquid. You need 330 ml (11¼ fl oz/1⅓ cups) liquid, so add water to make up the volume if needed.
Combine the strained liquid, tomato paste, sugar, salt and nutmeg in a saucepan. Bring to the boil over medium heat, then lower the heat and simmer for 5 minutes, stirring. Blend the cornflour with the vinegar. Pour this mixture into the saucepan and simmer for another 5 minutes, stirring.
Allow to cool before serving. Store in the refrigerator in a bottle.
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