1. Preheat oven to 400°.
2. Combine 1 cup water, 1 tablespoon salt, and sugar in a shallow dish. Add sablefish fillets; let stand 15 minutes. Remove fillets from brine; pat dry. Discard brine. Sprinkle fillets with remaining 1/4 teaspoon salt. Combine mustard and egg white, stirring with a whisk until blended. Brush mustard mixture over fillets.
3. Combine panko and rind. Press panko mixture evenly over top of fillets. Place fillets on a broiler pan coated with cooking spray. Bake at 400° for 10 minutes. Remove pan from oven.
4. Preheat broiler.
5. Broil fillets 3 minutes or until brown.