Place potatoes and onions on a baking sheet. Drizzle with half of the harissa mixture and sprinkle with salt and pepper. Toss to combine. Roast for about 20 minutes.
Remove pan from the oven and turn on the broiler. Score the skin of the fish and rub with remaining harissa mixture. Place the fish on top of the potatoes and onions and broil for about 6-10 minutes.