Boil the black gram in plenty of water until just past the al dente stage (they need to be quite soft to become as sticky as they should be for this daal). If you have a pressure cooker this will be much quicker.
In a large pan, heat the ghee (or oil for a vegan option) and add the mustard seeds. Wait for them to pop, and then add the cumin seeds, curry leaves and asafoetida.
Quickly add the tomatoes, garlic, ginger and chillies. Cook this mixture out on a medium heat for around 5 minutes. Add the turmeric and cook for a further minute.
Add the cooked black gram and mix thoroughly. Add around 500ml hot water and stir constantly for 1-2 minutes to create a creamy consistency.
Add salt and lemon and taste. Stir in the coriander and serve hot with lashings of butter or ghee, paratha or naan.