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1
Heat oil in a 4 to 5 quart heavy pot over moderately high heat until hot but not smoking, then cook the shrimp shells, stirring, for one minute. Add the water, onion halves, carrot, bay leaf, parsley, thyme, peppercorns and salt and simmer, uncovered, 30 minutes.
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2
Pour stock through a large sieve lined with a dampened paper towel into a large bowl, discarding solids. (Shrimp stock may be made 1 day ahead to this point.).
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3
Return stock to cleaned pot. Add shrimp and cook over high heat ( stock should be hot but not boiling) stirring frequently until shrimp are just about cooked through (2 minutes- no longer). Transfer the shrimp with a slotted spoon to a large shallow baking dish, spreading in 1 layer to cool quickly.
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4
Bring stock to a boil and reduce to about 1 1/2 cups. This should take 30 to 40 minutes, then transfer to a bowl and set aside.
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5
Stir together flour and 1 1/2 tbsp butter until a paste forms and set aside.
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6
Cook minced onion in 1 tablespoons butter in clean pot over moderately low heat, stirring, until the onion is softened, about 3 minutes. Add sherry and boil, until liquid is reduced to about 1/3 cup ( about 2 minutes). Add the shrimp stock and bring to a simmer, then reduce heat to low and whisk in the remaining 5 1/2 tbsp butter until just incorporated. Whisk in flour paste, bit by bit, and simmer sauce, whisking, until slightly thickened, about 2 minutes. Add lemon juice, shrimp and salt and pepper to taste and cook over moderate heat, stirring, until shrimp are just heated through, about 1 minute.
