Put fish, skin side down, on foil-lined rack of a broiler pan. Sprinkle with 1/8 teaspoon salt and 1/8 teaspoon pepper (total), then brush with 1 tablespoon nut oil.
Broil 4 to 5 inches from heat until just cooked through, 6 to 8 minutes.
Meanwhile, grate zest from orange with a Microplane and squeeze 1/4 cup juice.
Whisk together zest, orange juice, lemon juice, 1/4 teaspoon salt, and 1/8 teaspoon pepper, then add remaining 2 tablespoons nut oil in a slow stream, whisking. Stir in scallion.
Transfer fillets (without skin; it will be stuck to foil) with a metal spatula to plates, then drizzle with some of vinaigrette and sprinkle with nuts. Serve remaining vinaigrette on the side.
