Preheat the oven to 200oC/Gas Mark 6.
Make the dressing: Whisk the olive oil with the lemon juice, 1 ½ teaspoons of salt and some pepper, then stir in the chopped oregano or marjoram.
Pre-bake the potatoes: Cut the potatoes into 8-10mm thick slices and put them into a large roasting tin in which the fish can lay either lengthways or diagonally. Add 6 tablespoons of the dressing, the sliced garlic, anchovies and toss together well. Spread out in an even layer and roast in the oven for 30-35 minutes.
While this is cooking, prepare the fish: Slash the flesh of the fish 5-6 times on each side. Season the cavity with salt and pepper then stuff it with the lemon slices, bay leaves and lemon leaves if using.
Finally, bake the fish: Remove the tray of potatoes from the oven and release them from the bottom of the tin if they have stuck. Brush the outside of the fish with a little of the remaining dressing and season with salt and pepper. Put it on top of the potatoes and pour any remaining dressing into the fish’s cavity. Return the roasting tin to the oven and roast for another 35 minutes until the fish is cooked through and the potatoes are tender.