Peel and finely chop the onion and carrot. Trim the dry ends off the celery and finely slice it.
Warm a large pan over a medium-low heat for 1 min. Add the chopped veg, 1 tbsp olive oil and a pinch of salt and pepper. Fry for 8 mins, stirring occasionally, till the veg have softened but haven't picked up too much colour. If they start to brown, turn the heat down.
While the veg fry, peel and crush the garlic. Finely chop the chilli, flicking out the seeds and white bits for less heat. Put 2 tsp ras el hanut and the vegetable stock cube into a heatproof jug. Pour in 800ml boiling water and stir to mix.
Stir the garlic and chilli into the veg. Tip in the lentils. Pour in the spiced veg stock. Cover, turn the heat up and bring to a boil. Once it is boiling, turn the heat back down so the soup is simmering. Leave to simmer for 20 mins.
While the soup simmers, trim the dry ends off the cavolo nero. Finely slice it. When the soup has cooked for 20 mins and the lentils are soft, stir in the cavolo nero. Simmer without the lid on for 5 mins to just cook the cavolo nero.
Juice the lemon into the pan. Give the soup a stir and taste it. Add a bit more salt or pepper if it needs it. Serve the soup in warm bowls.