Heat the oil in a large saucepan over moderate heat. Fry the onions, ginger and garlic until the onions are translucent.
Add the tomato and stir-fry breaking it up with the spoon. Pour in 300ml/ ½ UK pint water and stir in the other ingredients.
Bring to the boil. Lower heat, cover and simmer for 30 minutes. Remove from heat and allow to cool. Pour into a blender or food processor and liquidize.
