1. Combine first 6 ingredients in a medium bowl. Add 1/4 teaspoon salt; stir well. Cover and chill 30 minutes.
2. Prepare grill.
3. Sprinkle fish with remaining 1/2 teaspoon salt and pepper. Place fish, skin side up, on grill rack coated with cooking spray; grill 3 minutes. Rotate fish one quarter-turn (45 degrees) for diamond-shaped crosshatches. Grill 3 minutes. Turn fish over; grill 4 minutes or until desired degree of doneness. Serve with sauce and dill sprigs, if desired.