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1
Bring a large pasta pot two-thirds full of water to a boil, and using tongs, transfer 2 of the crabs to the pot. Cook until their shells turn bright orange, about 3 minutes, then transfer the cooked crabs to a colander set in the sink and run cold water over them. Add the next 2 crabs to the pot and repeat until all the crabs have been cooked.
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2
As each cooked crab becomes cool enough to handle, slip your thumb beneath the place on its underside where the cape of shell tapers to a point, and lift the bottom shell up, pulling the entire carapace off the body of the crab. Clean out and discard the spongy-feathery gills on the body. Carefully pick out any orange roe you may see inside the body, and reserve. Then snap one of the smaller legs off the body of the crab to use as a pick, to tease out any roe in the hollows of the carapace that taper to a sharp point, and reserve. Repeat with the remaining crabs until all the roe has been picked. Then split the crab bodies and legs, pick the meat from them, and add it to the rest of the picked crab meat.
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3
Pour off all but 3 cups of the cooking water, and add the picked crab bodies, legs, and carapaces, then return to a boil. Let boil until reduced by a third, about 10 minutes, and strain the crab broth into a 4-quart saucepan or Dutch oven.
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4
Heat the broth over high heat until it simmers. Put the flour in a small bowl or ramekin, spoon 3 tablespoons of the hot broth into it, and whisk it to a smooth paste with a fork. Pour the remaining broth into a blender, add 1/4 cup of the crab roe, and liquefy until the roe is completely dissolved in the broth. With the blender running, add the flour paste to the broth.
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5
Return the Dutch oven to the heat and melt the butter over medium-low heat until it's frothy. Add the shallot, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Cook, stirring occasionally, until the shallot is soft, fragrant, and translucent, but not brown, about 4 minutes. Add the half-and-half, the broth mixture, and 1 tablespoon of the sherry, and cover. When the soup comes to a simmer, turn the heat to low, uncover, and simmer gently, stirring occasionally, until the soup has reduced by about one-sixth, 12 to 15 minutes.
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6
Add the crab meat and cook for 2 minutes over low heat, stirring to combine. Season the soup with salt and black pepper.
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7
Pour a teaspoon of sherry into each bowl before serving, and garnish each bowl with about 11/2 teaspoons roe and chives, if using. Serve with additional sherry-in a shot glass for each guest, or in a cruet for passing around the table-for those who might want more.
