Make prick marks all over the skin of the lamb with the tip of a knife, going no deeper than the fat itself. Sit it in a large shallow roasting tin and massage it with both salts (the finer grains will slip into the prick marks while the larger ones will cling to the skin) plus pepper. Rub with olive oil and tuck the thyme under the joint.
Boil the potatoes for 5 minutes. Drain, tip back into the pan, add a little oil, season and shake the pan a little to roughen their edges. Put them around the joint.
Roast the joint for 20 minutes then turn the oven down to 190C/fan 170/gas 5 and roast for your calculated time. Turn the oven back on high again for the last 10 minutes if the lamb hasn't browned up enough. Turn the potatoes two or three times while the lamb cooks. When it is ready, take the lamb out of the roasting tin, crumble over the thyme and leave the rest, loosely covered with foil, for about 20 minutes. If the potatoes aren't crips put them back in the oven for another 15 minutes or so.
To make the gravy, put the roastng tin on the hob, add a touch of oil if it needs it then scatter over the cornflour and cook, scraping up all the scrunchy sticky bits from the tray, to make a dark paste. Slowly add around 500ml of vegetable cooking water or stock, season, and scrape the bits from the botttom of the tin again. Allow to bubble up furiously until you have a good, smooth gravy.
Serve the lamb with buttered carrots scattered with chopped parsley, braised beans and roast or dauphinoise potatoes - or both, for a fantastic carb hit.View more Lamb recipes...