Begin preparing the lamb the night before you are going to roast it. Set out the boneless lamb leg on your cutting board. Trim away as much of the fat on the inside of the roast as you can, paying special attention to any large or hard pockets of fat.

Mix together all ingredients of the Greek herb rub.

Rub about half of the herb mixture onto the the inside of the lamb roast. Roll up the roast with the fat cap facing out and tie it up with butcher's twine. (Here is a tutorial.) Using a small, sharp knife cut slits into the lamb roast every inch or so and push a sliver of garlic into each slit until you have evenly filled the lamb with garlic. Now rub the rest of the herb rub over the lamb. Put the lamb in a casserole dish, cover it with plastic wrap and put in the refrigerator until it's time to start cooking.

About 3 hours before you want to eat, preheat the oven to 250F. Then toss the potatoes with the salt and olive oil. Transfer the lamb to a roasting pan and surround it with the potato slices.

Roast the lamb for 2 hours, or until the lamb reaches 130F. Then take the lamb out of the roasting pan and let it rest for 40 minutes. Leave the potatoes in the pan let them continue cooking.

After the rest time, remove the roasting pan from the oven and heat the oven to 500F. This will take a few minutes. Add the lamb back to the roasting pan and roast lamb and potatoes together for 12-15 minutes, until the lamb is crispy and browned.

To serve, cut off the butcher's twine and slice the lamb thinly. You can make a gravy with the pan drippings or serve the lamb with tzatziki sauce.