Peel and finely slice the onion. Trim and cut the carrot in half lengthways, then cut into thin slices on the diagonal. Slice the celery into thin diagonal slices. Peel and chop 1 garlic clove.
Crumble the stock cube into a heatproof jug. Pour over 750ml boiling water and stir to dissolve.
Warm a large pan over a low heat. Add 1 tbsp oil, the sliced veg and the garlic. Scatter in 2 tsp fennel seeds. Fry for 6-7 mins till the onion has softened.
Pour in the stock. Tie up the bouquet garni with heatproof string and throw in the pan. Or strip the leaves from the rosemary, sage and thyme and finely chop, then add to the pan with the bay leaf. Cover and simmer for 10 mins.
While the soup cooks, peel and grate or crush the remaining garlic clove. Zest the lemon. Warm 1 tbsp oil in a small frying pan. Add the garlic, lemon zest and breadcrumbs. Gently fry for 3-4 mins with a pinch of salt till the breadcrumbs are a deep golden colour.
Drain and rinse the borlotti and cannelini beans. Add to the soup. Cook for 5 mins. Remove a couple of ladles of the soup from the pan and blitz in a food processor or put in a bowl and blend with a hand-held blender. Add back to the soup – this will thicken it.
Meanwhile prepare the cavolo nero. Remove any thick stems and finely slice the leaves. Add the cavolo nero to the pan and cook for 2 mins.
Squeeze in the juice from half the lemon. Taste and season as necessary. Fish out the bouquet garni (if put in whole). Ladle the soup into warm bowls and scatter over the crispy lemon garlic crumbs. Serve with the zested lemon cut into wedges.