Clean the livers, removing fat, and connective tissue. Place in a nonreactive casserole or bowl with the white wine, peppercorns, and bay leaf and refrigerate, covered for 6 hours.
In a medium saucepan, heat olive oil and add the onions. Once browned, add a pinch of salt and enough water to cover the onions halfway, and bring to a boil. Reduce the heat to medium and continue to cook until the onions fall apart, about 20 minutes. Drain the onions and place in a blender or food processor to puree. Return the onion puree to the pot and stir in the chicken stock and 1 tablespoon butter. Season with salt and pepper, to taste, and keep warm.
Drain the livers and discard the bay leaf and peppercorns. In a 12 to 14-inch saute pan, heat the remaining 2 tablespoons butter over high heat until it foams and subsides. Add the livers and saute for 1 minute. Add the brandy and saute for 1 minute more. Serve immediately with the onion sauce and Horseradish Sauce served over.
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.
In a small bowl, combine the horseradish and apple, then stir in the vinegar. This may be stored for up to 1 week, tightly sealed, in the refrigerator.
Recipe copyright 2001, Mario Batali. All Rights Reserved