"This recipe is the most requested and loved recipe from my mother-in-law, who is the best cook on the face of the earth. It's our traditional Thanksgiving dressing. It's very rich and worth every calorie."
Place the duck in a 12-quart stock pot with enough water to cover the duck by 2 to 3 inches. Bring the water to a boil, reduce heat to medium-low, and cook the duck at a simmer until the meat is tender, about 1 hour. Remove the duck from the broth, allow to cool, and pull the meat from the bones in bite-sized pieces. Reserve the broth.
Preheat oven to 375 degrees F (190 degrees C).
Mix the cornbread crumbs, chopped biscuits, onion, salt, garlic salt, black pepper, and sage in a large roasting pan. Beat the eggs in a bowl; pour over the cornbread mixture. Add the chopped duck meat to the pan; stir to mix the dressing thoroughly. Stir the reserved duck broth into the mixture until the dressing is very moist (about the consistency of thick pancake batter).
Bake in the preheated oven until a spoon inserted into the center of the pan of dressing will stand up, 45 minutes to 1 hour.