Place the finished stock into a large pot and add the celery, carrots, black garlic, and salt. Bring to a simmer and cook for 30 minutes. Remove and discard the black garlic.
My Chicken Stock
Wash onions and quarter, leaving skins on. Washing garlic and cut horizontally, leaving peel on. Peel carrots and roughly chop. Put the chicken, onions, carrots, garlic, peppercorns, parsley, and 1 teaspoon salt in a large stockpot. Add just enough cold water to cover the chicken (about 4 quarts) and bring to a simmer over high heat; then lower temperature to keep the stock a gentle simmer. Cook, covered, until the liquid is pale, golden, and flavorful, 3-4 hours. Strain and reserve the schmaltz (chicken fat) that rises to the top when the stock is chilled.
In a bowl, beat the eggs. Add salt, schmaltz, cinnamon, matzo meal, and baking powder; mix well. Loosely cover and chill, 30 minutes.
Bring a large pot of salted water to a low boil. When the matzo ball mixture is chilled and set, tear off golf ball-sized pieces and shape into rounds using wet hands. (Be aware that the balls will double or triple in size when cooked!) Gently drop the matzo balls into the boiling water and cook, 30 minutes or until cooked through; you can cut one open to check doneness.
Assemble the soup: Once matzo balls are fluffy and tender, remove to individual serving bowls. Ladle broth on top of matzo balls, garnish with parsley sprigs, and serve.