Peel and rinse the onions, ginger and garlic. Slice the onions and roughly chop the ginger and garlic.
Put the chopped ginger and garlic in a blender with about ½ a pint of the water and blend until smooth.
Take a large saucepan and put in the onions, blended garlic and ginger and the rest of the water. Add the salt and bring to the boil.
Turn down the heat to very low and simmer with the lid on for 40-45 minutes. Leave to cool.
Once cooled, pour about half of the boiled onion mixture into a blender and blend until perfectly smooth (at least 2 minutes).
Pour the blended onion mixture into a clean pan or bowl and repeat with the other half of the boiled onions. Wash and dry the saucepan.
Reserve about 4 tablespoons of the sauce at this stage to use in cooking the meat in your chosen curry. Pour the can of tomatoes into the rinsed blender and blend until perfectly smooth (about 2 minutes).
Into the clean saucepan, put the oil, tomato puree, turmeric and paprika. Add the blended tomatoes and bring to the boil.
Turn down the heat and cook, stirring occasionally for 10 minutes. Add the onion mixture to the mixture and bring to the boil. Turn down the heat to a simmer.
When froth rises skim this off. Simmer for 20-25 minutes, stirring occasionally to stop the sauce from sticking to the bottom of the saucepan.
