Bring the oil to heat on high in a heavy bottomed pan. When it's hot, drop in the whole spices and as they sizzle up, toss in the sliced onions with half a teaspoon of salt. The salt will help the onions caramelize faster.
Saute the onions, stirring regularly, for 15 minutes. As they turn golden, spoon in the ginger and garlic paste, and sauté for another five minutes.
Now mix all the powdered spices, bar the garam masala, into the onion masala. Stir for a couple of minutes and then add the chicken pieces. Seal the chicken pieces coating them well for about five minutes. Chicken on the bone is the most traditional way to make a curry, and adds a lovely depth to the flavor of the dish. If you're stretched for time, skinned, boneless chicken thighs are a better alternative than chicken breasts, which are fairly impenetrable to spices.
When the chicken is ready, take the pot off the heat, stir well and mix in the Greek yoghurt. Place the pot back on the hob, add 4 tablespoons of warm water, mix well, cover and cook for 20 minutes stirring from time to time.
Blend 20 cashew nuts to a smooth cream with two tbsp Greek yoghurt and 2 tbsp water. Finally, lower the heat to a simmer, stir in the cashew cream, remaining salt and sprinkle a pinch of garam masala over the chicken pieces.
