Heat oil. Add kalonji (nigella seeds) and fry for a few seconds until they begin to pop.
Add chopped onion and brown on medium-low heat for 5-7 minutes.
Add crushed ginger and garlic and fry for 30 seconds.
Add ground spices (turmeric, red chili and coriander powder) and fry for 30 seconds – 1 minute until aromatic. Splash with water to deglaze the pan if necessary.
Add tomatoes and salt. Fry on high heat to soften the tomatoes, about 5 minutes.
Add potatoes and coat in masala for 2 - 5 minutes.
Add ½ - 1 cup water, just enough to cover the potatoes, and bring to boil. Lower heat and let the masala simmer in uncovered saucepan for 10 - 15 minutes or until potatoes are done.
Turn off heat and gently fold in diced cilantro. Place a tight fitting lid on the saucepan and let the potatoes steam for another 5 minutes.