Heat the oil in a large non-stick frying pan or casserole dish. Brown the chicken thighs over a high heat until brown. Set aside. Add the onion and mushrooms to the pan and fry for 2-3 minutes.
Add the Thai paste, tamarind paste, coconut milk, fish sauce, lime juice and zest, and sugar. Bring to the boil and boil for 2-3 minutes. Season well.
Add a little hot sauce to the slaked cornflour and tip into the pan with the sauce, stirring continuously while bringing it back to the boil. The sauce will thicken. Return the chicken thighs to the pan.
Cover the pan with a lid, transfer to the oven and bake for about 45 minutes or until the chicken is tender.
Sprinkle with the chopped basil or coriander, and serve hot.
This dish can be made completely and then kept covered in the fridge for up to 48 hours before heating thoroughly and serving.