Preheat oven to 220°C/450°F/Gas Mark 6. Place potatoes in a pot, pour over wine and water, cover and bring to the boil. Cook 6–8 minutes until almost done. Add tomatoes and peas and gently stir, then take off the heat.
Drizzle oil into a roasting dish (I find a ceramic dish works best for this), sprinkle with basil leaves, then lay fish in, skin-side up. Scatter mussels around the fish, and spoon potato mixture over it all. Season with salt and freshly ground black pepper. Cover dish with a lid or foil and bake 10 minutes. Remove lid and continue to cook until fish is done and mussels are open — discard any that don’t open. (Cooking time will depend on thickness of fish pieces.)
Serve hot from the oven in winter, or at room temperature on a balmy day.