Soak the baccala in the refrigerator for 2 days in several changes of water. Rinse and pat dry.
Bring the chicken stock to a boil, reduce to a simmer and add the baccala. Cook 20 minutes at a simmer. Remove the fish, add the garlic, olive oil, milk and salt and pepper to taste, stir well to combine and cook 5 minutes. Return the fish to the pot, cook for 5 minutes, then serve sprinkled with chopped parsley.
In a large, heavy-bottomed saucepan, heat the oil over high heat until smoking. Add all the chicken parts and brown all over, stirring to avoid burning. Remove the chicken and reserve. Add the carrots, onions, and celery to the pot and cook until soft and browned. Return the chicken to the pot and add 3 quarts of water, the tomato paste, peppercorns, and parsley. Stir with a wooden spoon to dislodge the browned chicken and vegetables bits from the bottom of the pan. Bring almost to a boil, then reduce heat and cook at a low simmer until reduced by half, about 2 hours, occasionally skimming excess fat. Remove from heat, strain, and press on the solids with the bottom of a ladle to extract out all liquids. Stir the stock to facilitate cooling and set aside. Refrigerate stock in small containers for up to a week or freeze for up to a month.
Recipe copyright 2000, Mario Batali. All Rights Reserved