Preheat the oven to 220°C/fan 200°C/Gas 7.
Put the potatoes in a medium saucepan and cover with cold water. Place over a high heat and bring to the boil. Cook for 10 minutes then take off the heat and drain well in a colander.
Tip the potatoes onto a large baking sheet and smash very lightly with a fork to lightly crush. Season with salt and plenty of ground black pepper and drizzle with 2tsp of the oil. Bake for 20 minutes until the potatoes are getting crusty around the edges.
While the potatoes are cooking, place the fish on a work surface and slash through the skin 3 times with a sharp knife. Mix the lemon zest and thyme leaves together and rub into both sides of the fish fillets. Season with a little salt and pepper.
Take the potatoes out of the oven and scatter with the spring onions and tomatoes. Toss together lightly. Make four gaps in the vegetables and place the fish fillets on the baking tray, skin-side up. Drizzle the fish with the remaining oil. Return to the oven for a further 10-15 minutes until the fish is cooked and the tomatoes have softened. Drizzle over the lemon juice and serve with freshly boiled runner beans.
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