01 - Set the oven temperature to 425°F.
02 - Pat the leg of lamb dry with paper towels and create deep slits all over using a sharp knife.
03 - Place garlic slices into the slits on the lamb.
04 - Combine rosemary, thyme, sea salt, pepper, olive oil, and lemon juice in a bowl. Rub the mixture thoroughly over and into the lamb.
05 - Place the lamb in a roasting pan and arrange the potatoes, carrots, and onion around it if using.
06 - Cook at 425°F for 20 minutes, then reduce heat to 350°F and roast for an additional 70 minutes or until a thermometer inserted into the thickest part registers 135°F for medium-rare.
07 - Remove from oven, tent loosely with foil, and let rest for 15–20 minutes before slicing.
08 - Carve the lamb and serve alongside roasted vegetables and pan juices.