This delicious recipe is courtesy of Ariane Daguin.
Place beans in a large bowl; cover with water and let soak overnight.
Drain beans and place in a large earthenware casserole or enameled cast-iron Dutch oven along with ventreche, garlic, 1 onion, and carrot. Wrap parsley, celery leaves, thyme, bay leaf, cloves, and peppercorns in a piece of cheesecloth and tie to form a bouquet garni. Place bouquet garni into the casserole and cover with 10 cups water.
Place casserole over high heat and bring to a boil. Reduce heat to a simmer and cook, stirring frequently, until beans are just tender, about 1 hour. Drain, discarding onion and bouquet garni, and return to casserole. Add remaining onion, duck legs, and tomatoes. In a medium bowl, stir together demi-glace with 4 cups water, and add to casserole. Season with salt and pepper and place casserole over medium-high heat; bring to a boil. Reduce heat to a simmer, and cook until duck legs are tender when pierced with a fork, about 15 minutes.
Drain mixture into a colander set over a bowl; reserve cooking liquid. Cut ventreche into 1/2-inch square pieces; set aside. Cut each duck leg in half at the joint; set aside.
Preheat oven to 325 degrees. Season beans with 1 teaspoon salt and pepper. Place half of the beans in the casserole. Add ventreche, duck legs, duck, and garlic sausages; cover with remaining beans. Pour reserved cooking liquid over top and drizzle with duck fat. Cover, and bake until heated through and bubbling, about 2 hours. Cassoulet may be cooled and refrigerated at this point for up to 3 days. Bring to room temperature before proceeding.
Increase temperature to 400 degrees. Uncover casserole, and bake until top is browned, about 20 minutes.