Peel and finely chop the onion. Trim the celery and finely slice it. Trim, peel and finely chop the carrot. Warm a pan for 1 min. Add ½ tbsp olive oil and the chopped veg. Season well with salt and pepper. Cover and gently cook for 10 mins, stirring occasionally, till the veg have started to soften.
While the veg sweat, peel and crush the garlic. Dice the tomatoes. Slice any really thick cores out of the cavolo nero. Shred the leaves. Dissolve the stock cube in 800ml boiling water (or heat up 800ml homemade stock). Drain and rinse the butter beans.
Stir the garlic, tomatoes and beans into the pan with a pinch of cayenne. Pour in the stock. Leave the lid off (to keep the cavolo nero green) and bring up to the boil. Turn the heat down a little and simmer for 5 mins till the cavolo nero is cooked.
Zest and juice the lemon. Swirl the lemon juice into the soup. Taste it and add more salt and pepper if you think it needs it. Ladle into warm bowls and top with the lemon zest to serve.