Heat the oil in a heavy pan.

Add the chopped onion and stir for a few minutes with the heat on high.

Add the ginger, garlic and green chillies.

Stir for 30 seconds then put the heat down to low

Cook for 15 minutes stirring from time to time.

Add the turmeric, cumin and coriander and cook still very gently for a further 5 minutes.

Take off the heat and cool a little.

Now put 120 ml cold water in a blender and add the contents of the pan.

Then add the tomatoes and whiz it all up.

Put the puréed mixture back into the pan and cook for 25 minutes on a medium heat.

You can add a little hot water if it starts to catch on the pan but the idea is to gently fry the sauce.

Store until needed.