These instructions should work with bone in or boneless rolled and tied leg of lamb. Butterflied legs would take a little less time. When measuring bone in legs, place thermometer in thickest part of leg.
Liberally rub with olive oil, then with salt, pepper, garlic, and rosemary.
For best results, we will need to sear the outside of the meat. You can either do this in the oven at 500 degrees or in a pan until all sides are browned. (Smaller roasts are easier to pan sear. The oven usually takes ~15-20 min to brown outside of the roast.)
Set oven at 325. If you browned lamb in oven, let some of the 500 degree heat out as you tent the lamb in foil to ensure evenness in cooking. Roast leg of lamb until internal temperature is 130-135 degrees. This should be a perfect medium rare. Go 135-140 if you like medium.
The time should be around 20 minutes per pound. (A 6lb roast takes about 2 hours, but we always suggest checking it 20-30 minutes before the calculated time. We can always cook it longer, but once its overcooked, it will never be as good!)
As for any roast of meat, always let sit for 10-15 minutes outside of oven before carving. Enjoy!