Soak the beans overnight. Drain and add to a pan with 2 litres of water and a strip of kombu. Bring to the boil and simmer for 30 minutes. Don't worry if they are not completely tender at this stage. Remove and discard the kombu.
Preheat the oven to 180C. On the hot plate add the olive oil to a flame proof dish and add the garlic and leeks and saute gently until soft. Add the beans and stock and stir through. Pierce the black limes a few times and add them to the bean mixture. Add the olives and carrots and cover with a lid.
Place the casserole dish in the oven for 1hr and 15 minutes. Remove from the oven and lay the flathead fillets on the bean mixture, cover and return to the oven for a further 5 minutes. While you are cooking the fish steam some English spinach and serve with the dish. Season with cracked black pepper.