Heat 1 tablespoon of the oil in a frying pan, add the mushrooms and cook for 5 minutes until softened. Stir in the hazelnuts, then remove from the heat and stir in the parsley.
Lay the plaice fillets on a clean surface and divide the mushroom mixture between them, then roll up to enclose the stuffing. Place each fillet on a piece of foil large enough to enclose it, sprinkle with the white wine, season with pepper, add a knob of butter and wrap well. Bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 15 minutes or until cooked through.
Heat the remaining oil in a frying pan, add the lettuce and fry for 2–3 minutes on each side.
Meanwhile, cook the potatoes in a saucepan of boiling water for 12–15 minutes until tender. Drain and season, then add the mint and lightly crush with a fork. Serve the potatoes topped with the lettuce and plaice parcels.