Put the lamb in a roasting pan. Peel the garlic and put it into a food processor with the cumin seed, mint leaves and lemon juice.
Add a generous grinding of salt and some black pepper. Blitz to a coarse paste, adding enough olive oil to make slush. You want it thin enough to be easily spread but thick enough to cling to the lamb.
Massage the roast well with the spice paste, spreading it over the skin and into the cut sides of the flesh.
Now set aside in a cool place, preferably not the refrigerator for an hour, basting occasionally with any of the paste that has run off.
Preheat the oven to 400 degrees. Roast the meat for 45 minutes to an hour, depending on how you like your lamb (45 minutes should give you a roast that is still pink and juicy inside). Remove from the oven and rest the meat for a good 10 minutes before carving.
To make a gravy, put the roasting tin over a moderate heat and pour in a large glass of wine, stock or water and bring it to a boil. Stir with a wooden spoon, scraping away at the pan to dissolve any stuck-on meat juices.
Let the gravy bubble a little, check it for seasoning -- it may need salt and pepper. Then keep it warm while you carve the lamb.