Basic Curry Sauce

INGREDIENTS:
3 tablespoons vegetable oil or ghee (clarified butter)
1 medium onion - finely chopped
4 cloves garlic - peeled and sliced
1.5 inch piece root ginger - peeled and thinly sliced (it should look about the same volume as the garlic)
(optional) 2 mild fleshy green chillies - de-seeded and veined then chopped
half teaspoon turmeric powder
half teaspoon ground cumin seed
half teaspoon ground coriander seed
5 tablespoons plain passata (smooth, thick, sieved tomatoes, US = purée) or 1 tablespoon concentrated tomato purée (US = paste) mixed with 4 tablespoons water

METHOD:
- 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!