Lightly dry roast the nigella seeds over gentle heat then set aside. Dry roast the cumin seeds over gentle heat until fragrant. Cool, then grind into a fine powder.
Heat the ghee in a frypan and cook the onion, ginger, garlic and chilli over moderate heat until they are softened and aromatic, about 5 minutes.
Stir in the nigella seeds, ground cumin, garam masala and ground cardamom and cook a few minutes until fragrant. Add the lentils and stir, then add the water and cook over moderate heat until the lentils have softened, about 30 minutes. Add more water if necessary.
Season with salt, lime juice and pepper. Stir in the coriander and tomato slices and serve.