Preheat the oven to 160C. Separate and peel the garlic cloves, and crush them with the salt in a pestle and mortar. You need enough salt to give the mixture some grit. Mash the garlic into the salt. Add the rosemary and thyme, then mix with enough olive oil to make a thick paste.
Rub the garlic mixture over the whole surface of the lamb. Place the lamb into a GN container.
Add 250ml of water to the roasting tin. Baste the meat with the liquid, then cover the lamb with foil. Roast for 3 hours, until cooked to your liking.
When cooked, pour off any excess oil from the top of the liquid, leaving the cloudy, herby sediment in place. This is the gravy.