Preheat the oven to its highest setting. Slash the fish to the bone 2 or 3 times on each side near the head end so they will cook evenly. Season inside and out with salt and pepper and place in an ovenproof dish.
Lubricate the fish with 50 ml of the olive oil and put into the hot oven. Turn the heat down to 200 C, 400 F Gas 6 and bake for about 25 minutes until the flesh is firm and beginning to flake from the bone.
Meanwhile chop the herbs roughly and mix them with the remaining olive oil and lemon juice to taste. Season with salt and pepper.
To serve place a large dollop of mashed potato on to warmed plates then carefully lift the fish fillets away from the backbone and lay them next to the mash.