Tip the lentils into a bowl. Cover with cold water. Put to one side to soak for 10 mins. Fill the kettle and boil it.
Peel and finely chop the onion. Peel and grate or crush the garlic. Peel and finely chop the carrots. Slice two thin slices from the middle of the lemon. Put to one side. Finely grate the zest from the rest of the lemon. Juice it.
Warm 1 tbsp olive oil in a pan. Add the onion and carrot. Season with salt and pepper. Fry over a low heat for 5-8 mins till the onion is soft but not coloured. Stir often. If it starts to brown, turn the heat right down.
Stir the garlic and lemon zest into the veg. Measure out 1 tsp of the ground cumin. Add a pinch of cayenne (it's spicy, so add as much or as little as you like). Cook and stir for 1 min.
Drain the lentils. Tip them into the pan. Measure out 1 tbsp tomato purée. Add to the pan. Pour in 1 ltr hot water from the kettle. Pop on the lid. Simmer for 15 mins till the lentils are soft.
While the soup simmers, slice any thick middle cores from the kale. Finely shred the leaves.
Add the kale to the soup. Simmer, without the lid on, for 5 mins till the kale is tender. Taste and adjust the seasoning. Ladle into warm bowls. Serve topped with the lemon slice and a pinch more cayenne if you like.