Set the oven to gas mark 4 or 180°C. Line a large roasting tin with 2 very large sheets of foil reaching over the sides. Put a sheet of Bakewell or greaseproof paper inside. Spread the sliced onions in the tin along with the halved garlic bulb. Pour in the wine.
Mix the oregano and oil with seasoning and rub the mixture all over the lamb. Put lamb on the bed of onions. Bring the paper and foil over to wrap the lamb in a loose parcel. Cook for 3 hrs.
Open up the parcel and stir the potatoes into the meat juices, then add the sliced lemon. Re-wrap the parcel and cook for another 1½ hrs.
Transfer the meat to a board or platter. Rest it for 10 mins. Sprinkle over capers, if using. Put potatoes and lemon on the platter, or in a separate bowl. Serve the meat juices as they are or thickened with cornflour, if you prefer. Serve the meat and potatoes with some spring greens drizzled with olive oil. (Not suitable for freezing).
Feature: Kate Moseley. Photos: David Jordan. Props stylist: Sue Radcliffe