Place the lamb in a non-corrosive ovenproof dish on a bed of the chopped onions. Whisk together the salt, pepper, lemon juice, garlic, olive oil, redcurrant jelly and 150ml water and pour it over the lamb. Season the lamb with a little salt, cover the whole thing closely with foil and put in the oven for 30 minutes while you prepare the potatoes.
In another non-corrosive ovenproof dish large enough to hold all the potatoes in a single layer and the lamb (which you will add later), tip in the potatoes and sprinkle them with the garlic and oregano and pour over the lemon juice and oil. Barely cover the potatoes with cold water and season with salt and pepper.
Place the potato tray in the oven when the lamb’s first 30 minutes has elapsed, and continue cooking for 15 minutes before reducing the oven temperature to 150C/ 140C fan/ gas 2, without opening the oven door. Cook for a further hour.
Turn over the potatoes in their liquid, lift the lamb to sit, other side up, on top of the potatoes. Spoon off and discard any visible oil from the surface of the meat juices then pour the remainder and the onions over the lamb and potatoes. Cover loosely with foil and return to the middle of the oven for a further hour (or more) or until the potatoes are soft and golden tinged brown and the lamb, very tender.
Serve cut into thick slices and spoon on some of the soft lemon potatoes and their delicious juices.