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
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!