Place butter in saute pan and cook until melted. Add the white wine add fish and saute until fish is done. Do not overcook. Just until the fish flakes easily.
Remove fish to serving platter. Add the chicken or fish stock and bring to a boil. Let cook until reduced a bit. Whisk in lemon juice. Mix water and cornstarch and whisk into sauce and cook until thickened.
Season with salt and pepper. Serve over fish and garnish with parsley.