Peel and finely slice the onion. Trim and cut the carrot in half lengthways, then thinly slice it on the diagonal. Slice the celery into thin diagonal slices. Peel and chop one of the garlic cloves. Dissolve the stock cube in a heatproof jug with 1ltr boiling water.
Warm a large pan over a low heat. Add 1 tbsp oil, the veg and garlic. Add 1 tsp fennel seeds (keep the rest for later). Season with salt and pepper. Fry and stir for 5 mins till the onion has softened.
Pour in the stock. Tie up the bouquet garni with heatproof string and add to the pan. Or strip the leaves from the rosemary, sage and thyme and chop them, then add to the pan with the bay leaf. Cover and simmer over a medium heat for 20 mins.
While the soup is simmering, tip the flour into a bowl. Peel and grate in the remaining garlic clove. Grate in the lemon zest. Add the remaining fennel seeds and a good pinch of salt. Pour in 1 tbsp olive oil and 2-4 tbsp cold water. Stir together with your hand till it forms a dough. Knead for a few mins till it's soft and elastic. Pop back in the bowl. Cover with a tea towel and set aside.
Drain and rinse the cannelini and borlotti beans. Add to the soup. Pop the lid back on. Simmer for 5 mins, then take off the heat. Trim the dry ends off the cavolo nero, then finely slice the leaves and stalks.
Divide the dough into two balls. Pat into rounds, around 1cm thick. Brush one side of each flatbread with 1 tsp olive oil. Set a frying pan over a mediumhigh heat. Place a flatbread into the pan and cook for 2-3 mins till golden. Brush the uncooked side with 1 tsp oil and flip over. Cook for another 2-3 mins.
Slide the flatbread out of the pan and wrap in a clean tea towel. Add the other flatbread and cook in the same way, brushing the uncooked side with 1 tsp oil before flipping it over.
Fish the bouquet garni out of the soup (if you put in whole). Add a couple of ladles of soup to a blender and whizz till smooth, or use a handheld blender in the pan to blitz the soup a little. This will help to thicken the soup up, but you can skip this step if you like.
Add the cavolo nero to the pan and simmer over a medium heat for 2-3 mins to wilt it. Squeeze in the juice from half the lemon. Taste the soup and season if it needs it. Ladle the soup into warm bowls. Serve with fennel and garlic breads and the remaining lemon half, cut into wedges.