Hearty duck meatballs add a delicious modern touch to classic matzo ball soup, perfect for lunch or dinner.
Preheat oven to 400 degrees. Season duck with salt and pepper. Transfer to a roasting pan. Add carrots, celery, and onions; season with salt and pepper. Roast duck and vegetables, turning occasionally, until duck is browned, about 1 1/2 hours.
Transfer duck and vegetables to a 12-quart stockpot, reserving fat. Pour fat through a sieve into a bowl; refrigerate, covered. Add 8 quarts cold water to pot. Add herb sprigs. Bring to a boil; cook 10 minutes. Simmer over low heat 1 1/2 hours.
Pour stock through a sieve into another large pot; reserve meat, and discard vegetables. Season stock with salt and pepper. Let cool. Skim off any fat, and add to reserved fat in refrigerator.
Remove meat from bones; discard skin and bones. Process meat, mushrooms, 2 eggs, the egg white, 1 cup matzo meal, the chopped herbs, 1/4 cup reserved fat, 1 tablespoon salt, and 1/2 teaspoon pepper in a food processor until a paste forms. Shape into 60 (1 1/4-inch) meatballs. Refrigerate, covered, until ready to use.
Melt 1/4 cup reserved duck fat in a small saucepan over low heat. Lightly beat remaining 4 eggs in a bowl. Whisk in melted fat, 1/4 cup water, and 1 teaspoon salt; season with pepper. Stir in remaining cup matzo meal. Refrigerate, covered, until slightly firm, 30 minutes to 1 hour.
Bring stock to a boil. Scoop out 26 (1 1/4-inch) balls of matzo mixture, and add to stock. Reduce heat; simmer 10 minutes. Add meatballs; simmer 10 minutes. Garnish with parsley.
If you have duck fat left over, reserve 6 tablespoons for the Matzo Farfel.