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 or until fragrant.
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 chili, tomatoes and salt. Fry on high heat to soften the tomatoes, about 5 minutes.
Add potatoes and stir well to coat in masala.
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 until potatoes are done. Check for salt and spice, and adjust as necessary.
Turn off heat and gently fold in diced cilantro. Top with lemon juice. Place a tight fitting lid on the saucepan and let the potatoes steam for another 5 minutes.
