Step 1. First make a spice-rub by measuring out:

3 cloves of garlic, crushed
2 teaspoons of dry oregano
2 teaspoons of dry thyme
2 teaspoons of dry rosemary
1/4 teaspoon of fresh ground black pepper

Put these ingredients into a small bowl and mix everything together

Now, for the liquid part of the marinade, measure out:

1/2 cup (4 oz.) of olive oil
1/2 cup (4 oz.) of lemon juice, fresh squeezed
2 cups (16 oz.) of dry red wine

The Boneless Leg of Lamb

The netting (or string) holds the lamb together in that nice shape. Do not remove it until after the lamb is roasted. 

Step 2. Rub the garlic dry-rub spice mixture all over the lamb.

Step 3. Put the lamb in a large plastic bag and set it on a dish. I used a two gallon sized zip-lock bag. 

Step 4. Pour the lemon juice, olive oil and red wine over the lamb (in the bag.)

Squeeze out as much of the air as possible from the bag, and seal it closed. As a precaution against the bag leaking, wrap a second bag around the marinating lamb.

Step 5. Set the bag in a dish and let the lamb marinate in the refrigerator for several hours or overnight -up to 24 hours.

Step 6. Roasting the Boneless Leg of Lamb

Remove the lamb from the refrigerator at least 1 hour before cooking it.

Preheat the oven to 450 F. degrees.

Snip the corner of the bag and squeeze the marinade out into the sink. It will no longer be usable.

Pat the lamb dry with paper towels.

Put 2 teaspoons of salt and 3/4 teaspoons of freshly ground pepper into a dish. Season the entire lamb with the salt and pepper. (You put the salt and pepper into a separate dish first for sanitary reasons. You would not want to be going back and forth between touching the raw lamb and your main salt and pepper holders.)

Place the lamb on a roasting rack in a roasting pan.

Insert an oven-safe meat thermometer into the thickest part of the boneless leg of lamb.

Set the lamb into the 450-degree oven and roast for 20 minutes.

After the 20 minutes, lower the temperature to 325º F degrees and continue cooking the lamb until the internal temperature is to your liking.

When the meat is cooked, remove it from the oven and let it rest for 10 minutes. The temperature will increase by about 5 degrees while the roast is resting out of the oven.

With a pair of kitchen scissors cut through the netting (or string) and remove it completely.

Slice the lamb into 1/2 inch thick slices across the grain.  

I roasted some carrots and small red potatoes separately and served them with this Boneless Leg of Lamb.

From: http://startcooking.com/blog/365/How-to-Cook-a-Leg-of-Lamb