Keep the netting on the lamb intact while you use a sharp small knife to poke 8 whole evenly spaced throughout the leg of lamb, about 1.5 inches deep. Push 1 tablespoon of the black pepper into the holes with your fingers and then rub the whole leg down with 1 tablespoon of olive oil. Then, puncture the whole garlic cloves in one or two places with a knife and then stuff them into the holes in the lamb (puncturing the garlic cloves will help their flavor to seep out more readily). Place the lamb in an oven bag, tie a loose knot at the top, and refrigerate overnight.

The next day, remove the lamb from the refrigerator about 3 hours prior to when you want to begin roasting it. In a small bowl mix together the oregano, salt, and remaining tablespoon of black pepper. Set aside. Rub the leg of lamb down with another tablespoon of the olive oil, then rub it down thoroughly with the spice mixture. Pour the lemon juice, minced garlic, and remaining olive oil into the oven bag and then place the leg of lamb inside. Tie a knot at the top and refrigerate for another 2 and 1/2 hours.

Remove the oven bag of lamb from the refrigerator and preheat the oven to 375 degrees Fahrenheit while the lamb warms to near room temperature. Open the bag and toss in the butter, then re-tie the bag in a tight knot. Place the oven bag in a roasting pan and then make 3 small holes, about 1 inch long, in the very topmost part of the bag so that the hot air can escape but the cooking juices won't be able to seep out of the bag.

Place the pan in the oven and roast for 20 minutes, then lower the heat to 325 and cook another 40 minutes, or until the internal temperature of the lamb reaches 140 degrees Fahrenheit for a medium-done lamb. However, it is important to note that if your lamb leg is larger than 3-4 pounds it will take longer to cook, and the shape of the leg will also affect the cooking time, as thicker short legs will take longer to cook than longer, thinner ones.