How to Make It
Combine 8 cups water, salt, and granulated sugar in a deep roasting pan large enough to fit fish; stir to dissolve salt and sugar. Add fish, and let stand at room temperature 1 hour. Remove fish from brine; rinse under cold running water, and pat dry. Place salmon, skin side down, on a parchment paper–lined baking sheet, and chill, uncovered, at least 1 hour and up to 3 hours.
Preheat oven to 450°F with oven rack in top third of oven. Stir together mustard and brown sugar in a small bowl. Brush glaze over flesh side of fish. Roast in preheated oven on top rack until fish is opaque around edges, about 8 minutes. Turn broiler to high, and broil until fish is cooked through and glaze is bubbly and browned, about 8 minutes. Let stand 5 minutes.
Try this method with any 1-inch-thick fillet of flaky fish.