Heat oil. Add chopped onion and brown on medium low heat for 5-7 minutes till soft.
Add crushed ginger and garlic and fry for 30 seconds.
Add ground spices and fry for 30 seconds – 1 minute till aromatic.
Add tomatoes and fry on high heat for 5 minutes.
Add potatoes and coat in masala for about 2 minutes.
Add ½ cup water, bring to simmer, lower heat and cover for 10 minutes.
Lift cover, add a splash of water to deglaze the pan if needed. Add kalonji/nigella seeds, and fry for 2 minutes.
Add diced cilantro and fry for 1 minute.
Place on serving platter and serve with poori.