To reduce cooking time soak the lentils overnight. In the morning drain and wash. This step is optional.
In a large pan add the spices, onion, lentils, tinned tomatoes and sweet potato. Add enough water to cover and bring to the boil. Cook until the lentils are soft and the sweet potato falls apart when you press it. Keep an eye on the pan 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 make a it a little less chunky.
Season with salt and pepper.