Place all the paste ingredients into a chopper and chop to a paste. Set aside.
Using your fingers, mix the tamarind pulp with the water, if using pulp. Strain into a bowl using a medium mesh sieve. Or using your fingers, pick out all the seeds and any fibrous bits and lose them, keeping the tamarind paste aside until needed.
Heat the oil in a large saucepan on medium high heat and sauté the onion rings for 1 minute.
Add the lemongrass stalks and fry for 30 seconds, then add the paste and fry for 2-3 minutes until you get a strong aroma of the paste.
Add all the chicken portions, and coat thoroughly with the spice paste.
Add the tamarind paste and salt, mix thoroughly and bring back to a boil. Lower the heat right down and simmer for 45-60 minutes until the chicken portions are fully cooked. The time will depend on the size of your chicken portions.
When the chicken is fully cooked, add the vinegar, stir, then check the seasoning. It should be perfect, but add more salt, if you think it needs it. You should have a thick curry with a full on flavour. If you like more sauce, add a little more water, perhaps 60-125ml (¼-½ cup), stir, and bring back to a simmer.
Scatter the fresh coriander (cilantro) if using and serve with rice.