Preheat oven to 200F

Stir together stock, Worcestershire sauce, lemon juice, mustard, Cognac, and sherry in a small bowl.

With a mallet or the flat side of a cleaver, pound meat between sheets of plastic wrap to a 1/4 inch thickness. Pat meat dry and season with salt and pepper. Heat oil and 1 tablespoon butter in a 10-inch heavy skillet over moderately high heat until foam subsides. Add steaks in 2 batches and cook, turning once, about 1 1/2 minutes total per batch for medium-rare. Transfer to a heatproof plate and keep warm in oven.

Cook shallots in fat remaining in skillet over moderately low heat, stirring, until golden, about 5 minutes. Stir in stock mixture and any meat juices accumulated on plate. Bring to a boil and boil sauce, stirring, until reduced by about half, about 3 minutes. Add parsley and remaining 2 tablespoons butter and cook, swirling skillet until butter is incorporated. 1 minutes.

Spoon sauce over steaks.