Preheat the oven to 220C/425F/Gas 7.
Rub the plaice with olive oil, then arrange the mushroom slices over the fish in a pattern to resemble fish scales.
Place the butter over the top and season with sea salt and freshly ground black pepper.
Place onto a baking sheet and transfer to the oven and bake for 4-5 minutes, or until the fish is completely cooked through.
Meanwhile, for the sauce, place the potatoes, leek, mushrooms and butter into a saucepan over a low heat and fry gently for 10 minutes, or until the potatoes are becoming tender.
Turn up the heat and add the wine. Simmer the mixture until almost no liquid remains, then add the chives and cream. Reduce the heat to simmer for 1-2 minutes.
To serve, spoon some of the sauce onto a serving plate, top with the fish and place the potatoes alongside side.