Marinate the chicken in all the ingredients listed for 20 minutes, refrigerated. Once ready to cook, take out the marinated chicken from the refrigerator & let sit on countertop.
Using a mortar & pestle, coarsely grind the coriander, black pepeprcorn & cumin.
In a wide mouthed pan, heat up the oil on high.If using mustard oil, heat it to a point when its slightly smoky.For other oils, heat up till you see ripples on the surface.
Once the oil is heated, reduce the heat to medium & wait for 2 minutes. Add the mustard seeds & let them crackle. About 5-7 seconds.
Add the chopped green chillies next & let them crisp up for 10-15 seconds. Tip :- Be extra careful, mustard seeds & chillies splutter a lot when added to oil.
Next, add the coarsely ground spices to the oil & stir for about 30-35 seconds or till you smell the aroma.
Adding marinated chicken to the pan and stir-fry over high heat for few minutes, tossing continuously.The idea is to lightly brown the outside of the chicken but still keep it juicy inside.
Lower the heat once all the chicken has been added. Add the leftover marinade(if any) & stir frequently. Cook on low heat till the oil separates on the sides of the pan and the chicken is cooked through. About 10-13 minutes. Chicken cooks fast because the pieces are go tiny! Taste and adjust the salt, toss well and remove.
Before serving,you could garnish with chopped cilantro.