In a pan, put all the ingredients of the curry base, mix well, put a lid on the pan and allow to simmer on low to medium heat for about twenty minutes, until all the ingredients, especially the tomatoes and onions are mushy.
Once all the ingredients are mushy, put the curry sauce in a blender and reduce it to a fine paste.
Pass the blended curry through a fine grain sieve, straining all the pieces that have not blended. This will create a very smooth curry cream, which can be kept on the side in a bowl.
Using the same pan, add two tablespoons of butter and allow to melt on low flame; once the butter has melted, add ½ teaspoon of chopped fresh ginger and 3 slit chilies. Sautee these together until the rawness of the spices has gone.
At this stage, add the blended curry sauce to the pan and mix well and add a little bit of water (100-200ml – I added it to clean the blender). Leave on low flame with a lid to simmer for 20 minutes.
While the curry sauce simmers away, grill the chicken on a griddle, using the cooking spray to help yourself; should it be preferred the chicken can also be cooked in the oven instead. Allow the chicken to cook halfway before turning; this will allow for nice grilling lines on it and the darker pieces will try to mimic the flavor that a tandoor oven offers.
Once the chicken is cooked thoroughly, set aside on a plate.