1. Combine first 3 ingredients in a bowl. Place fish, skin side down, in a 13 x 9–inch baking dish. Rub sugar mixture over fish. Cover and refrigerate 8 hours.
2. Preheat oven to 175°.
3. Wipe off remaining sugar mixture from fish with a paper towel. Coat a jelly-roll pan with cooking spray. Place fish, skin side down, in pan.
4. Bake at 175° for 1 hour and 10 minutes or until fish flakes easily when tested with a fork or until desired degree of doneness.
5. Combine mayonnaise and mustard; stir well. Serve mayonnaise mixture with fish.