To make the beurre blanc: Combine the shallots, wine, and vinegar in a skillet over medium-high heat. Bring to a boil, and cook until reduced by half, about 2 minutes. Remove from heat and stir in lemon juice. Pour off liquid, leaving shallots in pan. Whisk in butter one piece at a time, until sauce is thick and emusified (move pan on and off the heat to keep it warm, but not hot). Cover, and keep warm.
To cook the fish: Preheat oven to 450 degrees. Season fish with salt and pepper. Dredge in flour, tapping off excess. Heat oil in a medium skillet over medium heat. Add fish, skin-side down, and cook for 2 minutes. Turn, and transfer to the oven until cooked through, 5 to 7 minutes. Serve immediately drizzled with sauce.