Place the mung beans in a bowl and cover with cold water. Leave to soak for 5–6 hours or overnight. Transfer to a colander and rinse under cold running water. Drain and place in a medium saucepan with the measured water. Add the asafoetida and turmeric, and bring to the boil. Boil rapidly for 10 minutes, then reduce the heat to low. Simmer gently for 10–15 minutes, skimming off any scum that rises to the surface and stirring often.
Use a balloon whisk to whisk the mixture until fairly smooth. Add the spinach and stir to mix well. Stir in the tomatoes and cook over a medium heat for 10–12 minutes, stirring often. Remove from the heat and stir in the chopped coriander.
Heat the oil in a small frying pan over a high heat. When hot, add the cumin seeds, mustard seeds, chilli, ground coriander, ground cumin, garlic and ginger. Stir-fry for 30–40 seconds, then tip the contents of the pan into the dhal. Stir to mix well and season to taste. Serve hot.