Pre-heat the oven to 220°C/fan 200°C/gas 7. Cook the potatoes in boiling water for about 10 mins, cut into thick slices, then cool.
In a bowl, toss the potatoes with the olives, garlic, half the chopped parsley, half the olive oil, the lemon zest and salt and pepper. Tip this mix over the bottom of a medium gratin dish.
Season the fish and place the parsley stalks in the cavity. Lay the fish on top of the potatoes and drizzle with the rest of the olive oil. Bake for 15 minutes until the potatoes start to crisp up around the edges.
Pour the wine over, then return to the oven for 10 minutes more until the potatoes have browned and the fish is cooked.
Remove the dish from the oven, scatter over the rest of the parsley and bring the dish to the table. When you serve up, don’t forget the lovely white wine juices in the bottom of the dish.