Heat the oil in a nonstick pan. Add the mustard seeds and fry for 20 to 30 seconds, or until the mustard seeds start to pop. Caution: Keep the pan well away from your face and eyes.
Be careful of the popping seeds and the splatter of oil.
When the mustard seeds have popped, add the onion, ginger, and cashew nuts and fry for 4 to 5 minutes, or until the onions are golden brown. Add the garlic, green chiles, and salt, and fry for 1 to 2 minutes, stirring often, or until the garlic has softened. Add the chicken pieces, and fry for 5 to 6 minutes, or until golden brown. Add the yogurt, stir well and bring the mixture to a boil. Reduce the heat until the mixture is simmering, then cover the pan with a lid and simmer, stirring occasionally, for 10 to 15 minutes. Add the water, stir well, then cover the pan again, and simmer the mixture for another 15 minutes, or until the chicken is tender and the sauce has thickened. Add the remaining ingredients, stir well, and continue to cook for 2 to 3 minutes, or until the mixture has warmed through. Serve immediately.
*Can be found at specialty Indian markets.