How to Make It
In a medium bowl, cover the beans with 4 cups of water and let soak overnight. Alternatively, cover the beans with water and bring to a boil. Remove from the heat and let stand for 1 hour.
Drain and rinse the beans. Melt the butter in a large nonreactive saucepan. Add the onion, carrot and celery and cook over moderate heat for 4 minutes. Add the beans, thyme, oregano, bay leaf and chicken stock, cover partially and bring to a boil. Reduce the heat to low and simmer until the beans are tender, about 1 hour; some liquid should remain.
Roast the bell peppers over a gas flame or under the broiler until charred. Transfer to a bowl, cover and let steam for 15 minutes. Peel the peppers, discarding the cores, ribs and seeds. Cut the peppers into 1/2-inch dice and add them to the beans along with the olive oil and vinegar. Cook over moderate heat until warmed through. Stir in the rosemary and parsley and season with salt and ground pepper.
Preheat the oven to 450°. Heat the vegetable oil in a large, heavy-bottomed ovenproof skillet. Season one side of the tuna with salt and a generous amount of the cracked bell pepper. Add the tuna to the skillet and cook over high heat until just browned, about 2 minutes per side. Transfer the skillet to the oven and cook for about 6 minutes, or until the tuna feels firm but gives slightly; it should still be pink in the center.
Spoon the beans onto plates. Set the tuna steaks on top, peppered side up, and serve.