Preheat your oven to 275°F (135°C) and prepare a roasting pan with a rack if available.
Mix the minced garlic, ground cumin, chopped rosemary, olive oil, salt, and pepper in a small bowl to create a fragrant marinade paste.
Rub the marinade all over the boneless lamb leg, ensuring an even coating on all sides. Let it sit for at least 15 minutes to absorb the flavors.
Place the lamb on the rack in the roasting pan and insert a meat thermometer into the thickest part of the meat.
Transfer to the oven and slow roast for approximately 3 hours, or until the internal temperature reaches 135°F (57°C) for medium-rare, or your desired doneness.
During roasting, the meat will slowly develop a rich aroma and a golden brown crust will form on the exterior.
Once cooked, remove the lamb from the oven and tent loosely with foil. Rest for 15 minutes to keep the juices locked in.
Slice the lamb thinly against the grain, revealing its tender, juicy interior with visible marbling and even coloration.
Serve immediately, allowing each slice to showcase its juicy texture and aromatic herbs.