Rinse the turbot and make two shallow diagonal cuts across the top skin. Season lightly with salt and set aside.
If using a stovetop-safe baking dish, heat a drizzle of extra virgin olive oil over medium heat. Add the sliced onions and sauté until translucent. Meanwhile, peel and slice the potatoes into rounds about 0.5–1 cm thick.
Add the sliced potatoes to the softened onions and cook over medium-high heat for 5–8 minutes (they do not need to fry through, as they will finish cooking in the oven). Preheat the oven to 180°C (350°F) using conventional top and bottom heat.
In a mortar, combine the garlic cloves, parsley, and a pinch of salt. Pound into a paste, then stir in a splash of the white wine and a drizzle of olive oil. Set aside.
Arrange the turbot directly over the layer of potatoes and onions in the baking dish. Pour the remaining wine and water around the fish, spoon the garlic-parsley mixture over the top, and finish with a light drizzle of olive oil.
Bake at 160°C (325°F) for 25 minutes, then increase the temperature to 210°C (410°F) and bake for an additional 10–15 minutes until the fish is cooked through and the potatoes are tender.
