03While pork stands, pour off and discard any fat from skillet, then add pomegranate juice to skillet and boil over moderately high heat until reduced to about 2/3 cup, about 3 minutes (if side of skillet begins to scorch, reduce heat to moderate). Stir together cornstarch and water and whisk into juice, then boil sauce until thickened slightly, 1 to 2 minutes.
04Remove from heat and add Sherry vinegar to taste, then swirl in butter until incorporated. Pour sauce through a fine-mesh sieve into a bowl and skim off any fat. Season with salt. Slice pork and serve with sauce.