Seafood: Shrimp in Sherry Butter Sauce

Serves 8 as main course
1 T veg oil
3 lb. med shrimp in shell, rinsed, peeled, and shells reserved
8 cups water
1 onion, halved plus 1/2 cup minced onion
1 carrot, halved
1 Turkish or 1/2 California bay leaf
2 parsley sprigs
1 fresh thyme sprig
1/2 t whole black peppercorns
1 1/2 t salt
1 1/2 T flour
1 stick unsalted butter, cut into T pieces and softened
1/2 cup med-dry sherry
2 t fresh lemon juice

Heat oil in 4-5 qt. pot over mod high heat until hot but not smoking, then cook shrimp shells, stirring, 1 min. Add water, onion halves, carrot thru salt and simmer uncovered 30 mins.
Pour stock through large sieve lined with dampened paper towel into 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, abt 2 mins. Transfer shrimp w a slotted spoon to large shallow baking pan, spreading in 1 layer to cool quickly. Boil stock until reduced to abt 1 1/2 cups, 30-40 mins; transfer to a bowl.
Stir tog flour and 1 1/2 T butter until paste forms; set aside. Cook minced onion in 1 T butter in cleaned pot over mod low heat, stirring until softened, abt 3 mins. Add sherry and boil until liquid is reduced to abt 1/3 cup, abt 2 mins. Add shrimp stock and bring to simmer, reduce heat to low and whisk in remaining 5 1/2 T butter until just incorporated. Whisk in flour paste, bit by bit, and simmer sauce, whisking, until slightly thickened, abt 2 mins. Add lemon juice, shrimp, and salt and pepper to taste and cook over mod heat, stirring, until shrimp are just heated through, abt 1 min. (Shrimp can be cooked in stock and stock reduced 1 day ahead. Cool stock, chill shrimp and stock separately, covered).