Finely chop the white onions, garlic, celery and spring onion.
Finely slice the red onion.
Wash and cut cavalo nero leaves into 1 cm ribbons and finely chop the stems.
Melt the butter in a pot and gently fry the onion, garlic and celery until soft.
Add the soaked and drained beans, vegetable stock, salt & pepper and baking soda. The baking soda reduces the cooking time of the beans. Cover and cook for about 30 minutes or until the beans are soft and can be broken when pressed between your fingers.
While the beans are cooking, heat the oil in a small pan and fry the red onion slices until golden. Drain and set aside to use as garnish.
Mix in miso and crush the beans with the back of a ladle to thicken the liquid.
To finish, add the cavalo nero, juice of half a lemon and the spring onion.