In a large saucepan, heat 3 cups of chicken stock. In 2 batches, coarsely puree the roasted vegetables and the chicken stock in the bowl of a food processor fitted with the steel blade. Pour the soup back into the pot and season, to taste. Thin with more chicken stock and reheat. The soup should be thick but not like a vegetable puree, so add more chicken stock and/or water until it's the consistency you like.
Place the chickens, onions, carrots, celery, parsnips, parsley, thyme, dill, garlic, and seasonings in a 16 to 20-quart stockpot. Add 7 quarts of water and bring to a boil. Simmer uncovered for 4 hours. Strain the entire contents of the pot through a colander and discard the solids. Chill the stock overnight. The next day, remove the surface fat. Use immediately or pack in containers and freeze for up to 3 months.
Preheat the oven to 425 degrees F.
Cut the carrots, parsnips, sweet potato, and butternut squash in 1- to 1 1/4-inch cubes. All the vegetables will shrink while baking, so don't cut them too small.
Place all the cut vegetables in a single layer on 2 sheet pans. Drizzle them with olive oil, salt, and pepper. Toss well. Bake for 25 to 35 minutes, until all the vegetables are tender, turning once with a metal spatula.
Sprinkle with parsley, season to taste, and serve hot.
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