Heat the oil in a pan over a medium heat and add the garlic. It is very important not to burn the garlic, so watch the pan and move the slivers around in the pan until they become soft and are just beginning to brown.
Now add the onion and fry for about 3 minutes until soft and translucent, sprinkling a little salt over the onions to help release moisture.
Stir in the garlic and ginger paste and chillies, and fry for about 20 seconds. Increase the heat to medium-high, add the ground spices and tomato purée and sizzle for a further 30 seconds, stirring continuously.
Add about a ladle or two of the base curry sauce and bring to a rolling simmer. (You don’t need to stir the sauce unless it is obviously catching on the pan.)
Be sure to scrape any sauce that caramelises to the sides of the pan back into the sauce. Add more base as required. If the curry is looking too dry, add more base. If too saucy, just cook it down.
Add the pre cooked chicken and coat with the sauce
Continue simmering until fenugreek leaves and check for seasoning, adding salt to taste.
Sprinkle with the chopped coriander (cilantro) and a few more chillies if you like. A twist of lime juice is a nice addition.