Put the fish (2 medium black bream (or bass), scaled and gutted) in a long container. Whisk together the marinade ingredients (plus extra olive oil to drizzle over the fish), pour over the fish and leave for at least 30 minutes and up to two hours. Meanwhile, make the stuffing. Put the rice and water in a small saucepan and simmer, covered, on very low heat for 10 minutes - the rice should almost cook through. Let it cool down a little, then mix with the remaining stuffing ingredients (on preserved lemon, flesh only, finely chopped; onion and celery stick, finely chopped and crushed garlic). Set aside Preheat the oven to 220C/425F/gas mark 7. Peel the potatoes and cut into 0.5cm thick slices. Put in a saucepan, cover with water and boil for about six minutes, to soften. Drain, refresh under cold water and leave to dry. Lift the fish from its marinade and transfer to a plate. Put the cooked potatoes and slices of lemon (thinly sliced, pips removed) into the marinade, toss gently to coat, then spread evenly inside a roasting tray that's large enough to accommodate the fish. Bake for 12 minutes, until the potatoes begin to take on some colour or the lemon starts to dry out. In the meantime, fill the fish with the stuffing. Lay this on top of the hot potatoes, drizzle with oil and sprinkle a couple of tablespoons of water over the potatoes. Turn down the oven to 200C/400F/gas mark 6. Cover the tray with aluminium foil, then return to the oven for 30 minutes. Remove the foil and cook for five to 15 minutes more, depending on the size of the fish. It's done when the stuffing is hot and the fish flesh bounces back when you press it. Serve straight from the tray, with extra lemon wedges, if you wish, though it's pretty lemony as it is.