For the Dal

1. Put the dal in a bowl and cover it with plenty of water. Stir it with your hands to loosen the starch, then drain and repeat until the water runs almost clear (4-5 times). Cover again with fresh water and leave to soak until needed. 
2. Heat the oil in a large casserole over low heat (to use a pressure cooker, see Note in book). Peel and finely chop the onion and saute until lightly golden. Meanwhile, peel and finely grate the ginger; trim and discard the lower stems of the coriander and chop the rest. 
3. Stir in the salt and cook the dal, covered, for 5 minutes more, stirring halfway through, then use a potato masher to break down the dal until slightly thickened. If necessary, adjust the consistency by adding more water or cooking it a little longer. Turn off the heat, then stir in the lemon juice, fenugreek leaves, if using, and most of the coriander. 

For the Tempering

1. For the tempering, peel and finely grate or crush the garlic and have all the remaining ingredients ready. Heat the oil in a small pan over medium heat, then fry the cumin seeds and mustard seeds until they begin to sizzle (if your pan is larger, occasionally angle it to one side to puddle the oil and fry everything evenly). Stir in the garlic and cook for 1 minute more until aromatic but not browned. 
2. Add the dried chillies and curry leaves and cook for a few seconds more until the leaves look vibrant green and slightly crispy. Turn off the heat and add the chilli powder and asafoetida. Stir well to heat through the spices, then pour it over the dal. Stir it through, then divide among bowls, sprinkle over the remaining coriander, and serve.