Lightly score fat on lamb in a crisscross pattern at 1" intervals. Place lamb in a large roasting pan and season generously with salt and pepper, coating evenly on all sides. Sprinkle garlic and rosemary all over lamb and drizzle with oil and lemon juice.
Using your hands, rub marinade into meat, making sure it's completely coated. Arrange fat side up in pan. Cover very tightly with foil and chill for at least 2 hours or, preferably, overnight (the lamb will develop more flavor the longer it marinates).
Preheat oven to 325. Roast lamb, basting occasionally and adding water by 1/4 cupfuls if pan is dry, until meat is completely tender and pulling easily away from the bones, 4-4 1/2 hours (be sure to secure foil tightly around roasting pan each time you finish basting the lamb).
Remove lamb from oven and uncover. Preheat broiler. Broil lamb, basting occasionally, until fat is golden brown and crisp, 5-7 minutes.
Transfer lamb to a serving platter. Spoon fat from surface of juices in pan; discard fat. Pour juices into a pitcher and serve alongside lamb.
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