Sprinkle the curry masala over the chicken and give a good stir round to coat the chicken.Leave the chicken to marinate for at least an hour or overnight in the refrigerator. Put the tamarind in a heat proof bowl or jug and pour over the hot, boiled water. Leave for at least 30 minutes, breaking it up and giving it a squish every now and then. Once you have a nice dark brown liquid, strain the liquid through a sieve into a bowl and discard the leftover pulp.
Heat the oil in a pan. When hot, add the onions. Cook on a moderate heat, stirring as needed, until starting to brown.
Add the garlic and ginger and cook for a further 30 seconds.
Add the marinated chicken thighs. Cook on a moderate heat until well-sealed all over. stirring regularly so the spices don't burn. If it looks like the spices are catching add a splash of water as needed.
Once the chicken is sealed, add the tamarind paste and just enough water to come 1/2 way up the chicken. Turn the heat down to low and put a lid on the pan. Cook very gently for 15 minutes.
Take off the lid, add the salt and cook for another 15 minutes with the lid off.
After 15 minutes the chicken should be cooked, and you can serve the curry. If the sauce is a little thin for your liking or you like your chicken falling off the bone cook for a little longer.
Traditionally there is no fresh coriander added to this dish but as you can see from the photos, I've added some. For me a curry isn't a curry without a handful of coriander!
This is a curry that's best served with plain rice so that it can absorb the flavour of the tasty gravy. Some sort of bread is recommended too to mop up all the sauce.