Soak the dal: Wash chana dal and soak in water for 2–3 hours. It should soften but retain a slight bite - you want texture, not mush. Drain completely.
Grate the mango: Peel and grate on a coarse grater. Squeeze out excess juice if very wet.
Mix: Combine drained dal, grated mango, coconut, green chillies, and coriander. Add salt and jaggery if using. (Taste: it should be tart, slightly sweet, nutty, and fresh.)
Make the tadka: Heat oil, add mustard seeds and wait for them to splutter. Add cumin, curry leaves, hing, and dried red chilli. Sizzle for 30 seconds.
Pour tadka over the dal mixture and toss well.
Serve immediately at room temperature or slightly chilled.