Place the soaked, drained beans in a saucepan, cover with cold water and bring to the boil. Cook on a medium boil for 10 minutes. Drain and set aside. Heat the oven to 180°C/gas 4.
Place the onion, garlic, carrot, leek and the 2 tbsp olive oil in a cast-iron casserole and heat until the contents start to sizzle. Fry for about 5 minutes, then add the diced boudin sausage and cook for 3 more minutes.
Add the stock and the beans to the casserole dish. Bring to the boil and add the sprig of thyme. Season with sea salt and freshly ground pepper. Cover and transfer the pot to the preheated oven. Cook for about 45 minutes until the beans are tender. Remove from the oven but leave the oven on.
Brush the fish fillets with a little olive oil and season. Place in an ovenproof dish and bake, uncovered, in the oven for 10-12 minutes until the fish is just firm to the touch but not overcooked. Remove and keep warm.
Place the bean pot back on the stove, add the vinegar and cook briskly, uncovered, for about 10 minutes until the liquid is reduced by a third. Remove the beans and vegetables from the broth with a slotted spoon and place on warmed serving plates.
Whisk the butter into the broth until it turns glossy, then stir in the parsley. Spoon the sauce over the beans and place the baked turbot on top. Drizzle around the basil oil.
Recipe Courtesy of James Martin: