Wash fish and pat dry. Sprinkle all over with salt. Wrap in 2 layers of aluminum foil. Fold over and seal seam and ends. Place on a cookie sheet.
Bake at 375° F for 40 to 45 minutes or until fish flakes easily. Meanwhile, melt butter in a small sauce pan. Stir in cornstarch, mustard and dill. Gradually add apple juice. Stir until smooth and thickened. Add the diced apple and lemon juice.
Open foil packet, remove skin from fish, and transfer to a platter. Serve sauce over fish.Servings: 4