Boil, steam or microwave potato until tender; drain.
Meanwhile, using mortar and pestle, lightly crush chickpeas.
Heat oil in large frying pan; cook onion, stirring, until soft. Add seeds, turmeric, chilli, leaves and crushed chickpeas; cook, stirring occasionally, over medium heat 10 minutes. Add potato and buttermilk; cook, covered, over low heat 5 minutes. Stir coriander into the potato masala.
Meanwhile, make mint rasam.
Warm chapatis, one at a time, in large heated frying pan; place a quarter of the potato masala on each chapati then roll to make filled masala dosa. Serve with mint rasam and chutney.
Dry-fry lentils, seeds and chilli flakes in small frying pan, stirring, until fragrant. Using mortar and pestle, crush mixture finely. Place tomato, green chilli, mint and lentil mixture in medium saucepan; cook, stirring, 5 minutes. Add extra lentils, tamarind and the water; bring to the boil. Boil, uncovered, stirring occasionally,
about 10 minutes or until lentils are soft and liquid has almost evaporated.