Preheat the oven to 180ºC/350ºF/Gas Mark 4.
Add the lentils to a large saucepan, cover with cold water, bring to the boil then simmer for 30 minutes or until the lentils are soft.
Put the potatoes on a roasting dish, drizzle with olive oil and season. Roast them in the oven for 25 minutes.
Warm a glug of oil in a frying pan, add the onions and sweat on a low heat for 15 minutes.
When the onion is soft add the garlic, ginger and chilli to the frying pan and cook for two minutes.
Add the spices and cook for a further one minute.
Add the onion and spice mix to the lentils along with the coconut milk and tin of tomatoes, mix well and simmer for ten minutes.
Stir in the sweet potato and leave to stand for 5 minutes then season with the lime juice and salt and pepper.
Ladle the dahl into bowls and top with the cherry tomatoes, a sprinkling of coriander and a lime wedge.
Serve on its own or even better, mop up all the juices with sourdough or naan bread.