Cook the 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 nigella seeds for 1–2 minutes or until the spices are sizzling. Add the chopped chillies, tomatoes, lentils and tindori and bring to the boil. Cover the pan, reduce the heat and simmer gently for 10 minutes, stirring occasionally.
Mix the 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 serve with chapatis.