Place the lentils in a saucepan and add enough water to barely cover them. Bring to the boil and then simmer for about 10 minutes, until they are soft but not mushy. Drain well and set aside.
Heat the oil in a large non-stick frying pan. Add the leek, chillies, curry leaves and mustard seeds, and cook over a medium heat for 4-5 minutes.
Add the mango, tomatoes, chilli powder, ground coriander and chicken. Cook for 8-10 minutes, stirring occasionally. Add the cooked lentils and cook for a further 2 minutes, or until the chicken is cooked through with no pink showing.
Garnish with fresh coriander before serving with plain basmati rice or naan bread.
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