Heat the olive oil in a large (12-inch) saute pan and add the pancetta. Cook over medium heat, stirring often, until the fat is rendered and the pancetta is golden brown and crisp, 5 to 10 minutes. Remove the pancetta to a plate lined with a paper towel.
Add the Brussels sprouts, salt, and pepper to the fat in the pan and saute over medium heat for about 5 minutes, until lightly browned. Add the raisins and chicken stock. Lower the heat and cook uncovered, stirring occasionally, until the sprouts are tender when pierced with a knife, about 15 minutes. If the skillet becomes too dry, add a little chicken stock or water. Return the pancetta to the pan, heat through, season to taste, and serve.
Place the chickens, onions, carrots, celery, parsnips, parsley, thyme, dill, garlic, salt, and peppercorns in a 16 to 20-quart stockpot with 7 quarts of water and bring to a boil. Skim the surface as needed. Simmer uncovered for 4 hours. Strain the entire contents of the pot through a colander, discarding the chicken and vegetables, and chill. Discard the hardened fat, and then pack the broth in quart containers.
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