Slow-roasting Trout is a sure-fire, never-fail technique for tender, flaky fish. Coating it with harissa paste ensures it’s flavourful too.
Preheat oven to 275°F. Whisk oil, harissa, and garlic in a medium bowl. Pour half of harissa oil into a 2.5-qt. baking dish and swirl to coat. Thinly slice a lemon half and remove any seeds; scatter slices in dish.
Season Trout on all sides with salt and place in dish. Pour remaining harissa oil over Trout, spreading evenly over flesh with a pastry brush or spoon. Roast 15 minutes. Remove from oven and baste fish with harissa oil pooled in dish. Return to oven and continue to roast until flesh flakes apart easily with a spoon but is not quite cooked through, 10–20 minutes longer.