Fry the onions, ginger/garlic paste, then add the spices: turmeric, chile powder and coriander. Stir for about 30 seconds, then add the tomato paste. Mix well together then add the cashew milk, water and cook through for about 5 minutes or so. You can now add the chopped chiles and the kasuri methi (fenugreek leaves), mix together, and you´ve got your base.
The following day, marinate the chicken according to the recipe, and cook in a little oil until just white. While you´re waiting for the chicken, gently warm up the base and add the yoghurt, little by little, incorporating it into the sauce. Do NOT let the base get too hot or the yoghurt will separate. Once your chicken is done and the yoghurt incorporated, add the other ingredients and cook through until done.
NOTE: I cook a lot of Mughlai dishes and the first move is often to soak some raw cashew nut in a small glass of water for about 30 minutes, fry up the onion till it´s soft, then blend the cashews and the onion.