Marinate lamb for at least an hour in a blend of yogurt and ginger and garlic pastes.
Heat oil in a deep heavy-bottomed pan and add nigella and cumin seeds. Once they start to sizzle, add in sliced onions and fry till lightly browned.
Stir in spices and tomato paste, and add in the marinated lamb along with the yogurt.
Cook covered on low heat for 35-40 minutes till lamb is tender, adding water in between if it starts to dry up and stick to the pan. Season with salt and add in sliced peppers, letting it cook for another 5-10 minutes.
Garnish with chopped coriander leaves and serve warm.
Notes
I would recommend using a Dutch Oven for this recipe.