Put the toor dal, turmeric and water in a heavy-based saucepan and boil for 45–60 minutes or until the peas are soft when pressed between the thumb and index finger.
Meanwhile, use a mortar and pestle to pound the garlic, ginger and a good pinch of salt to a paste. Heat the oil in a small frying pan over low heat. Add the mustard seeds and allow them to pop. Add the cumin seeds, dried chillies, curry leaves, chilli powder, asafoetida and ginger and garlic paste and fry until fragrant. Stir into the dhal.
Add the chopped tomato and simmer for another 10 minutes. Season to taste with salt and garnish with coriander leaves.