SHRIMP IN SHERRY BUTTER SAUCE

Steps:

• Heat oil in a 4- to 5-quart heavy pot over moderately high heat until hot but not smoking, then cook shrimp shells, stirring, 1 minute. Add water, onion halves, carrot, bay leaf, parsley, thyme, peppercorns, and salt and simmer, uncovered, 30 minutes.
• Pour stock through a large sieve lined with a dampened paper towel into a large bowl, discarding solids. Return stock to cleaned pot. Add shrimp and cook over high heat (stock will be hot but not boiling), stirring frequently, until just cooked through, about 2 minutes. Transfer shrimp with a slotted spoon to a large shallow baking pan, spreading in 1 layer to cool quickly. Boil stock until reduced to about 1 1/2 cups, 30 to 40 minutes, then transfer to a bowl.
• Stir together flour and 1 1/2 tablespoons butter until a paste forms, then set aside. Cook minced onion in 1 tablespoon butter in cleaned pot over moderately low heat, stirring, until softened, about 3 minutes. Add Sherry and boil until liquid is reduced to about 1/3 cup, about 2 minutes. Add shrimp stock and bring to a simmer, then reduce heat to low and whisk in remaining 5 1/2 tablespoons 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.