Cook the rinsed lentils in a saucepan of boiling water for 20 minutes until soft. Drain well.
Meanwhile, heat the oil in a large saucepan and fry the turmeric, garam masala, cumin seeds and black onion seeds for 1–2 minutes or until the spices are sizzling and the mustard seeds begin to pop.
Add the chopped chillies and the tomatoes together with the drained lentils and tindori. Cover the pan and simmer gently for 10 minutes, stirring occasionally. Mix the brown sugar and tamarind paste with the boiling water and add to the pan. Stir well and simmer for a further 5 minutes. Season to taste and allow to cool. Cover and chill until required.
Shred the mango finely and mix with the red onion and coriander leaves. Cover and chill until required.
When ready to serve, reheat the curry gently over a medium heat until piping hot. Top with the green mango and red onion mixture and serve with chapatis.