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1
In a large kettle or stockpot combine the carcass with 4 1/2 quarts of the water, or enough to cover it, and simmer the mixture, uncovered, for 3 hours.
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2
Strain the stock through a large sieve into a large bowl, return it to the kettle, and boil it until it is reduced to about 10 cups.
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3
In a blender puree the garlic and the gingerroot with the remaining 1/3 cup water.
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4
In a heavy kettle heat the oil over moderately high heat until it is hot but not smoking and in it cook the puree, stirring, for 2 minutes, or until the liquid is evaporated.
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5
Add the potatoes, peeled and cut into cubes, the onion, the carrots, and 5 cups of the stock and simmer the mixture, covered, for 30 minutes, or until the vegetables are very soft.
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6
In the blender puree the mixture it batches until it is smooth, transfering it as it is pureed to another large kettle.
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7
Stir in the remaining stock, the coconut milk, the lime juice, and salt to taste, simmer the soup for 10 minutes, and stir in the chopped coriander.
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8
The soup may be made 2 days in advance, cooled completely, uncovered, and kept covered and chilled.
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9
The soup keeps, covered and frozen, for 2 months.
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10
Serve the soup garnished with coriander sprigs.
