Press the SAUTE button in your instant pot and set it to LOW.
Dump the milk, ghee, ginger garlic paste, spices (except the chaat masala) and the tomato puree together and give everything a good whisk.
Simmer at LOW (Saute mode) for 2-3 mins until the sauce becomes smooth and silky.
In the meantime make a smooth paste of the birista (fried onion) and the cashews. Set aside.
Add the onion + cashew paste, the chaat masala and about 250-300 ml warm water to the simmering sauce. The water amount may vary. The trick to judge the water is that after adding the chicken pieces they should just dip in the sauce and not remain under the sauce.
Press the CANCEL button and than close the lid of the IP, seal the valve to the SEALING position.
Now press the PRESSURE COOK button and set to at LOW for 1 min. OR you may totally skip the pressure cooking part and instead simmer the sauce at LOW (SAUTE mode) for 6 mins stirring frequently.
After 1 min as the IP starts to beep do a QPR (Quick pressure release), open the lid and give the sauce a good stir.
Now add the cream, kasturi methi and half of the chopped ginger to the sauce. Add the roasted chicken pieces and mix everything well.
At this point its VERY IMPORTANT to check that the chicken should just dip in the sauce and not be completely under the sauce. Refer the pic No. 16 and check the video. If need be add some warm water.
Close the lid of the IP. Seal the valve to the sealing position and press the PRESSURE COOK button and set it at LOW and set the time for 10 mins.
After 10 mins when the IP starts to beep do not do a QPR instead wait for a natural pressure release for at least 8 mins, release the remaining residual pressure and open the lid. Or you might instead do a complete NPR and allow the pin (the small round structure just beside the valve) to drop naturally.
The chicken will be completely cooked and reach that ultimate tender state where the meat will fall off from the bones. Serve it with Naan bread or rice.