Cut the chicken into bite sized pieces, wash and leave to drain on a colander. When the water has drained away, transfer it to a bowl and add the rest of the ingredients mentioned under 'For the marination', mix well with the chicken and keep it aside for 15-20 minutes.
Once the chicken is marinated, shallow fry it in a frying pan in batches. Make sure not to crowd the pan. Alternatively you can airfry the chicken at 200 degrees C for about 10-12 minutes or till done.
Heat oil in a wok/kadai and fry the onions till golden brown. Remove them and add the cashew nuts to the same oil and lightly fry them till golden. Remove. Let both these ingredients cool down a bit, then using a little water (approx 2-3 tablespoons) grind them to a fine paste along with the red chilli powder & cumin powder. Remove into a bowl and rinse the mixer jar with a little water (1/4 cup) and set aside
In the same wok, heat some more oil if required and lightly fry the bell peppers till they look slightly translucent and lose most of their crunch. Remove and set aside
Back into the wok, add the chopped garlic and fry lightly, do not burn it. Then add the cubed onions and fry till translucent. Add the tomatoes and a little salt and fry till the tomatoes turn slightly mushy.
Now add the ground onion & cashew nut paste and fry for a few minutes. Add the red chilli powder, Add the reserved water from the mixer jar and adjust the consistency of the gravy.
Add the shallow fried or airfried chicken pieces and give everything a good mix. Check the taste and add more salt, achar or red chilli powder if required. Add the fried bell peppers and let the gravy simmer for half a minute.
Turn off the heat & serve hot with chapathis, rotis or rice