Cut the potatoes into 1 cm (½ inch) thick slices – you can peel them first if you wish to, but this isn't necessary. Cut into chunky chips. Mix 2 tablespoons of the oil with the tapenade, chilli powder and plenty of salt. Toss in a bowl with the potatoes until evenly coated.
Tip the potatoes into a shallow ovenproof dish or roasting pan and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 30 minutes until pale golden, turning the potatoes once or twice during cooking.
Meanwhile, score the fish several times on each side. Mix the remaining oil with 1 tablespoon of the herbs, the garlic and a little salt and pepper. Pack the bay leaves, lemon slices and remaining herbs into the fish cavities and lay the fish over the potatoes in the dish, pushing the potatoes to the edges of the dish.
Brush the garlic and herb oil over the fish and scatter the olives over the potatoes. Return to the oven for a further 30 minutes until the fish is cooked through. Test by piercing the thick end of the fish with a knife; the flesh should be cooked through to the bone.