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Heat the oven to 220C/200C fan/gas 7. Simmer the potatoes in a large pan of boiling salted water for 5 mins until just tender. Drain and set aside.
Slash the flesh of the fish three times in a criss-cross pattern on both sides. Rub with the olive oil, then season with sea salt and black pepper. Pop the bashed garlic cloves and lemon slices into the cavity of the fish. Lay the potatoes and all the tomatoes in the bottom of a baking dish and season. Lay the fish on top and cook for 20-25 mins, or until the thickest part of the fish is cooked through (the flesh should come away from the bone easily and look opaque). Remove the fish, cover to keep warm and cook the potatoes and tomatoes for 10 mins more.
To make the salsa, tip the watercress, herbs, chilli and capers into a food processor and pulse to finely chop, drizzling in a bit of the extra virgin olive oil as you do. Mix in the remaining oil, then the garlic and lemon juice, and season to taste. Put the veg on a serving platter, sit the fish on top and drizzle with the salsa to serve.