Fold the fillets in half, skin side in. Place the fillets in the prepared dish with the wine, onion, lemon slices, thyme, and peppercorns. Cover the dish loosely with foil and bake for 10 to 12 minutes for small fillets and 16 to 18 minutes larger fillets, or until the fish is just cooked through. Carefully lift the fish from the pan and place on a warm serving dish. Strain any cooking liquids and reserve. Discard the solids. To make the sauce, melt the butter in a medium saucepan over medium heat. Whisk in flour and cook until the roux is bubbly and golden brown. Remove the pan from the heat and pour in the strained cooking liquids. Stir well. Return the pan to the heat, add the half-and-half, and simmer the sauce for 3 minutes, stirring constantly. Season with the salt and white pepper. Add the grapes and lemon zest. Spoon sauce over the sole and serve immediately.