1. Heat the oil in a large saucepan and gently fry the onions for 5 minutes until lightly browned. Add the crushed garlic and fry for another minute. Stir in the curry paste and stock, and bring to a gentle simmer.
2. Add the sweet potatoes and cook for 5 minutes. Stir in the lentils and tomato purée, cover with a lid and cook very gently for a further 15-20 minutes, stirring occasionally, until the lentils and sweet potatoes are tender but not completely collapsing.
3. Meanwhile, cut away the skin from the mango and discard the stone. Roughly chop the flesh and put it in a food processor with the salad onions, dates, coriander and lime juice, then blend until slightly pulpy but not completely puréed. Transfer to a small serving bowl.
4. Stir a dash of water into the dahl if the mixture is very thick, then ladle into shallow dishes. Serve with the chutney and naan bread.