Preheat the oven to 425 degrees. Melt the butter gently in a saucepan over medium heat, then stir in the Chardonnay, mustard, capers, and lemon juice. Heat about 5 minutes to burn off the alcohol.
Add the dill. Place the fish in a baking pan, skin-side down. Pour the sauce over the fish and bake for 20 minutes or until the fish is flaky.