Preheat oven to 425° F.
Trim as much of the outer fat from the shank as you can, set the fat aside, and then rub the shank with the lemon juice. Cut open 4-5 slits in the shank, located a few inches apart. Insert a clove of gresh garlic into each slit you cut.
Put the prepped shank into a roasting pan, and surround with the onion, which has now been sliced. Season generously with pepper and salt, then finish and rub in olive oil.
Put the seasoned shank and sliced onion into the oven in a baking pan, and cover for a half-hour. After cooking for a half-hour, remove the cover and continue cooking for one hour, or until lamb reaches an internal temperature of about 130 degrees. and is golden brown on the outside.
To finish, place the shanks on a high-temp grill about 2 to 3 minutes each side for extra-nice golden crispiness on the outside – just enough to sear in the fats. Allow meat to sit 10 minutes before serving.