Preheat the oven to 200C/400F/Gas 6.
For the sweet potato chips, place the parboiled sweet potato into a roasting tin, drizzle over the olive oil and season, to taste, with salt and freshly ground black pepper.
Scatter over the rosemary or thyme, and roast in the oven for 15-20 minutes, or until golden-brown and crisp.
Meanwhile, for the sauce, blanch the spinach and watercress in a pan of boiling water for thirty seconds, then drain and refresh in ice-cold water. Drain well.
Chop the spinach and watercress and mix together in a bowl with the parsley, tarragon, crème fraîche and lemon zest. Season, to taste, with salt and freshly ground black pepper.
For the halibut, heat an ovenproof frying pan over a high heat.
Drizzle the oil over the fillet and season, to taste, with salt and freshly ground black pepper. Place into the hot pan, skin-side down, and cook for 3-4 minutes or until the skin turns golden-brown and crisp. Turn the fish and transfer to the oven to roast for 8-10 minutes, or until cooked through.
For the mushrooms, heat the oil in another medium-sized frying pan over a high heat, add the chopped garlic and mushrooms, and fry for 6-8 minutes, or until golden-brown and softened. Season, to taste, with salt and freshly ground black pepper and the lemon juice.
To serve, place the fish onto a serving plate with the chips on the side. Spoon the mushrooms on top and to the side of the fish, followed by the sauce, and place one of the roasted lemon halves on the side.
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