How to Make It
Heat oven to 425° F. On a rimmed baking sheet, toss the potatoes with 1 tablespoon of the oil, the chili powder, ½ teaspoon salt, and ¼ teaspoon pepper.
Roast, tossing once, until golden brown and tender, 20 to 25 minutes.
Meanwhile, place the fish and scallions on a second rimmed baking sheet. Drizzle with the remaining tablespoon of oil and season with ½ teaspoon salt and ¼ teaspoon pepper.
Using a vegetable peeler, peel strips of zest from the lemon. Thinly slice them and sprinkle on the fish; reserve the lemon.
After the potatoes have cooked for 10 minutes, place the fish in the oven and roast until it is opaque throughout and the scallions are tender, 12 to 15 minutes.
Cut the reserved lemon in half and squeeze over the fish. Serve the fish and scallions with the potatoes.