Heat the oil in a wide, heavy pan over high heat. Add the chicken and sear for 5 to 6 minutes, turning, until lightly browned.
Add the cumin seeds, half the ginger, and the garlic. Stir for 30 seconds.
Add the tomatoes. Cook over medium-high heat, stirring and mashing, for 10 to 12 minutes, until they break down completely and the oil starts to separate from the sauce.
Add the chili powder, turmeric, salt, and green chilies. Cover, reduce the heat to medium-low, and cook for 20 to 25 minutes, stirring occasionally, until the chicken is cooked through (bone-in is best near 175°F) and the sauce is thick. Add a splash of water if it starts to stick.
Uncover and cook 2 to 3 minutes to tighten the sauce. Garnish with the remaining ginger and cilantro.