Preheat the oven to 220 C, Fan 200 C, Gas 7. Heat the olive oil in a large frying pan. Add the onion and garlic and cook over a low heat until soft.
Add the potatoes and stir to coat in the oil, then stir in the tomatoes and wine. Cover and stew gently until the potatoes are tender. Season with salt and pepper.
Transfer the mixture to a baking dish large enough to hold the fish. Make 2 or 3 slashes to the bone on each side of the fish at the plump end near the head. Season the fish inside and out and place on the potatoes.
Scatter the olives around and drizzle over a little extra olive oil. Bake for 20-30 minutes until the fish is done.