1 pound dried Great Northern beans
4 or 5 boned duck breast halves (2 to 2 1/2 lb. total)
About 2 tablespoons salt
2 tablespoons sugar
1/4 pound thick-sliced bacon, chopped
2 onions (about 1 lb. total), peeled and chopped
5 cloves garlic, peeled and minced or pressed
3 cups fat-skimmed chicken broth
3/4 cup dry red wine such as Beaujolais Nouveau or Zinfandel
1 firm-ripe tomato (about 1/2 lb.), rinsed, cored, and chopped
1 tablespoon chopped fresh thyme leaves or 1 teaspoon dried thyme
1/2 teaspoon pepper
1 dried bay leaf
1/2 pound garlic sausages, sliced 1/2 inch thick
Thyme sprigs, rinsed
Sort beans and discard debris. Rinse beans, drain, and put into a 5- to 6-quart pan with 4 quarts water. Bring to a boil over high heat, cover, reduce heat, and simmer for 1 hour. Drain beans, discarding liquid; if making up to 1 day ahead, cover and chill. Put beans in a 41/2-quart or larger electric slow-cooker.
Meanwhile, pull and cut skin from duck breasts; reserve skin for cracklings (cover and chill up to 1 day). Place breasts in a bowl. Add 2 tablespoons salt and the sugar, and mix; cover and chill at least 30 minutes but no longer than 1 hour. Rinse breasts well under cool running water; if making up to 1 day ahead, cover and chill. Cut meat into 1/2-inch chunks.
In a 10- to 12-inch frying pan over medium-high heat, frequently stir bacon until browned and crisp, about 5 minutes. Discard all but 1 tablespoon bacon drippings. Add onions and garlic to bacon; stir often until onions begin to brown, 8 to 10 minutes. Add broth and wine; bring to a boil over high heat, scraping up browned bits from pan bottom. Pour onion-broth mixture into slow-cooker with beans. Add tomato, chopped thyme, pepper, and bay leaf; mix. Place duck pieces on the beans.
Cover and cook until beans and duck are very tender to bite, 5 to 6 hours on low, 4 to 5 hours on high. If there is more liquid than desired, uncover slow-cooker, turn heat to high, and simmer to concentrate the mixture, up to 30 minutes.
Stir sausage slices into cassoulet and cook until hot, 5 to 10 minutes. Ladle cassoulet into bowls, sprinkle equally with duck cracklings, and garnish with thyme sprigs.