Arrange a couple of large sheets of parchment paper on a work surface. Set the leg of lamb on top of the parchment and cut small slits all over the leg meat. Nestle the garlic and rosemary into the slits, letting the rosemary stick out a bit, studding the meat. Whisk the extra-virgin olive oil and lemon juice in a small bowl and rub it all over the lamb. Season, liberally, with salt and pepper, to taste, then rub the lamb with the thyme and parsley. Cover the leg with another sheet of parchment paper, and bring the bottom pieces up to meet with the top piece, forming a packet. Secure with kitchen string and marinate in refrigerator for 3 to 4 hours.
Preheat the oven to 300 degrees F. Put the lamb into a roasting pan and rest the leg bone on 1 corner of the pan to settle juices under the leg as it roasts. Roast the lamb for 3 hours, then remove it from the oven and let it rest for a 1/2 hour. Unwrap and carve 2/3 of the meat. Arrange the lamb on a serving platter and spoon the pan juices over the slices. When you remove the lamb switch oven to 375 degrees F to bake off other make-ahead meals, such as the Stuffed Peppers with Lamb and Eggplant.
Serve roast lamb with Mint Sauce, Roasted Tomatoes and Asparagus alongside, if desired.
After your main meal is carved, cut the remainder of the meat into 1/2-inch slices. Cover and refrigerate.