4 (6-ounce) pieces arctic char fillet with skin
3 tablespoons pistachio or pecan oil, divided
1 navel orange
1 tablespoon fresh lemon juice, or to taste
1 scallion, thinly sliced
2 tablespoons chopped pistachios or pecans
                    

Preheat broiler. 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.