- Heat the oil in a heavy pan then add the chopped onion and stir for a few minutes with the heat on high. Add the ginger, garlic and green chilli (if using). Stir for 30 seconds then put the heat down to very low.
- Cook for 15 minutes stirring from time to time making sure nothing browns or burns. Add the turmeric, cumin and coriander and cook, still very gently, for a further 5 minutes.
- Don't burn the spices or the sauce will taste horrid - sprinkle on a few drops of water if you're worried.
- Take off the heat and cool a little. Put 4 fl oz cold water in a blender, add the contents of the pan and whizz until very smooth. Add the passata and stir.
- Put the puréed mixture back into the pan and cook for 20 - 30 minutes (the longer the better) over very low heat stirring occasionally.
- You can add a little hot water if it starts to catch on the pan but the idea is to gently "fry" the sauce which will darken in colour to an orangy brown.
- The final texture should be something like good tomato ketchup. Warning - it WILL gloop occasionally and splatter over your cooker, it's the price you have to pay!