Soak the lentils in plenty of water overnight. This step is optional, but does reduce cooking time.
Wash the lentils well in a sieve or colander.
To a large pan add the spices, onion, washed lentils, tinned tomatoes and sweet potato. Add enough water to cover and bring to the boil. Turn down the heat and simmer until the lentils are soft and the sweet potato falls apart when you press it. Keep an eye on the pan as it bubbles away, and add water if it starts to dry out.
Add the kale and tomato purée and more water if necessary. Cook until the kale wilts (about 5 minutes), then take off the heat.
Using a hand blender, blitz the soup to break it up and thicken it. Don’t overdo it though you don’t want it smooth.
Season with salt and pepper.