Preheat oven to 400°. Prepare fish: Put fish into a deep baking dish. Dissolve salt and sugar in 2 qts. cold water and pour enough over fish to cover. Brine, chilled, 20 to 30 minutes. "Brining seasons the fillet all the way through," says Clark. Remove from brine and pat dry with paper towels. With a very sharp knife, score skin several times to keep it from shrinking and thereby buckling the fillet as it cooks.
Combine maple syrup and orange zest and pour into a shallow dish just large enough to hold fish. Arrange sablefish skin side up in glaze and let marinate 20 minutes at room temperature.
While fish is marinating, prepare vegetables: Toss sweet-potato cubes with olive oil, season with a little salt and pepper, and roast in a single layer on a rimmed baking pan, turning once, until browned and tender, 15 to 20 minutes. Set pan of potatoes aside (keep oven on).
Meanwhile, blanch beans in boiling salted water just until tender-crisp, about 3 minutes. Drain, then submerge in a bowl of ice water to stop cooking. Drain again and add to pan of potatoes.
Make herb sauce: Purée ingredients in a blender.
Finish fish: Blot excess glaze with paper towels and lightly oil skin sides. Place large cast-iron or other pan (don't use nonstick) over medium-high heat on stovetop until very hot, 2 to 4 minutes. Add fish, glaze side down, and cook just until the glaze has browned and created a thin crust, 15 to 30 seconds. Immediately turn fish skin side down and sear until sizzling and crisp, 3 to 4 minutes.
Put in oven to roast until opaque in the center, 3 to 4 minutes. While fish is roasting, slide pan of sweet potatoes and beans into oven to reheat.
For each serving, make a small, tight stack of green beans in center of plate and drop in some sweet-potato cubes and blackberries. Top with fish and drizzle some herb sauce around it. Season with salt.