A holiday open house is the perfect opportunity to serve this hearty, robust French dish.
Evenly divide chicken stock and beans between 2 large stockpots. Bring both pots to a boil over medium-high heat. Reduce heat and let simmer for 20 minutes, skimming foam from surface as it rises. Remove pot from heat and let beans cool in liquid.
Preheat oven to 450 degrees. Fit 2 roasting pans with racks; set aside.
Remove thyme leaves from 3 sprigs of thyme. Season ducks, inside and out, with salt, pepper, and thyme leaves. Place two ducks on one rack in a roasting pan, and one duck on another. Add enough water to each roasting pan to come just below the bottom of the racks. Transfer both roasting pans to oven and roast for 45 minutes.
Fit another roasting pan with a rack. Season goose, inside and out, with salt, pepper, and sage. Using a fork, prick skin all over and place on rack in roasting pan. Add enough water to come just below the bottom of the rack. Transfer to oven; roast for 1 hour.
Place lamb and pork bones on a large rimmed baking sheet. Season with salt and pepper; transfer to oven and roast for 30 minutes.
Meanwhile, melt 1/4 cup duck fat in a large skillet over medium-high heat. Add lamb cubes, working in batches as necessary, and cook, turning, until browned on all sides; transfer to a plate and set aside. Add pork cubes, working in batches as necessary and adding 1/4 cup more duck fat, if needed, 1 tablespoon at a time (you may not need to use the full 1/4 cup). Cook, turning, until browned on all sides; transfer to a plate and set aside. Add pancetta to skillet, working in batches as necessary, and cook, stirring occasionally, until browned on all sides; transfer to a plate and set aside. Alternatively, melt 1/4 cup duck fat in each of 3 large skillets and, working in batches, brown meat separately in each skillet.
Drain fat from skillet; add 1/2 cup of duck fat to skillet and melt over medium-high heat. Add onions and garlic to skillet, in batches if necessary, and cook, stirring, until golden, about 10 minutes. Add tomatoes and 1 bottle of wine; let simmer for 15 minutes. Alternatively, if using 3 skillets, divide onion, garlic, tomato, and wine evenly among the 3 skillets. Transfer to stockpots with beans and their liquid, dividing evenly.
Remove breast meat from ducks and goose. Remove the legs of each and cut each leg into two pieces; remove bones from thighs. Cut breast and thigh meat into bite-size pieces and set aside. Remove wings and set aside along with drumsticks. Cover meat, wings, and drumsticks and refrigerate overnight. Using a cleaver, halve carcasses; set aside.
Place 2 large layers of cheesecloth on work surface. Repeat with another 2 layers of cheesecloth to make 2 separate cheesecloth stacks. Evenly divide duck and goose carcasses, roasted bones, parsley, leeks, carrots, cloves, bay leaves, and remaining 6 sprigs thyme between cheesecloth stacks. Wrap and tie with kitchen twine to enclose. Add one to each pot with beans and add enough water to cover beans by 2 inches. Place pots over medium-high heat; simmer until beans are almost tender, about 1 1/2 hours. Beans should be covered by 2 inches of liquid at all times; if liquid begins to reduce, add water as necessary. Remove from heat and remove as much excess liquid from beans as you can; transfer liquid to an airtight container and refrigerate overnight. Let bean mixture cool for 1 hour; transfer to refrigerator and refrigerate, uncovered, overnight.
The next day, preheat oven to 350 degrees. Place beans over medium-high heat; bring to a simmer. Remove cheesecloth packets and discard. Skim fat from the surface of reserved bean liquid and divide evenly between the two pots of beans.
In a Dutch oven that holds at least 18 quarts, alternately layer the beans with remaining bottle of wine and a mixture of the reserved cubed lamb, pork, pancetta, duck, and goose, seasoning between each layer with salt and pepper. Top the mixture with the reserved duck and goose drumsticks and wings.
Make the Topping: In a medium bowl, mix together breadcrumbs, parsley, garlic; season with salt and pepper. Drizzle with duck fat and stir to moisten. Sprinkle evenly over to cover. Transfer Dutch oven to oven and bake until a light-brown crust has formed, 2 to 3 hours.
About 1 hour before you are ready to serve, place sausages in a high-sided skillet; add enough water to skillet to cover sausages by three-quarters. Bring water to a simmer over medium-high heat; cook, turning, about 35 minutes. Cover skillet and continue cooking until sausage is cooked through, about 10 minutes more. Remove sausage from casings and cut on the bias into 1/2-inch slices. Top cassoulet with garlic sausage and serve.