Prep Garlic Rosemary Rub: whisk together Garlic Rosemary Rub ingredients in a small bowl.
Prep Lamb: Cut 10-12 1-inch slits in the lamb and push a slice of garlic into each one. Rub all over with the Garlic Rosemary Rub. Let marinate at least 2 hours or overnight.
Prep Mint Yogurt Sauce: combine Mint Yogurt Sauce ingredients in a medium bowl. Cover and place in the refrigerator for later.
Room Temp + Prep Grill: Leave lamb out and allow it to come to room temperature before grilling (if you are only marinating your lamb for 2 hours you can simply leave it on the counter and let it come to room temp while it marinates). Season lamb with salt and pepper. Preheat grill to 350℉. and ensure the grill grates are clean.
Grill Lamb: Place prepared lamb on the grill grates over indirect heat and close the lid (no need for a drip pan, there won't be many drippings). Keep grill temp between 325-350 degrees F. Let cook until the internal temperature of the lamb reaches 128 degrees F. for a medium-rare finish (use a meat thermometer).
Rest: Remove from grill and let rest 10 minutes before slicing - the internal temp will continue to rise as it rests reaching between 130-135℉ at the time of slicing.
Notes
Lamb Temps: medium-rare 130-135 degrees F.; medium 135-140 degrees F.; medium-well 140-145 degrees F.
Cook Time: there are a lot variables that go into total grill time such as thickness of lamb, outdoor temp, wind, etc. That being said,15-20 minutes per pound is pretty usual, but track the internal temperature with an instant read thermometer or probe to be sure.