Step 1

Heat the oven to 200°C/180°C fan/gas Scatter the potatoes, lemon slices, onion, half the tarragon and the garlic over the base of the dish or tray. Drizzle with the olive oil and season well. Cover loosely with baking paper (to prevent the food colouring too quickly), then roast for 30 minutes or until the potatoes are tender, turning everything once or twice. Remove from the oven (see Make Ahead).

Step 2

Turn the oven up to 220°C/200°C fan/gas Slash the fish 4-6 times on each side, then lay it on top of the potatoes and lemons. Chop the remaining tarragon, mix it with the softened butter, then spread this mixture over the fish and season with salt and pepper.

Step 3

Return the dish/tray to the oven and roast for 18-20 minutes until the fish is just cooked through. Check by inserting the tip of a sharp knife into one of the slashes and teasing the flesh from the bone. If it comes away with ease, it’s cooked. Take the whole tray to the table to serve.