Mix remaining ingredients and pour over lamb, coating it thoroughly (make additional marinade if needed).
Marinate lamb 6 hours or preferably, overnight.
Drain marinade and allow lamb to warm (remove a lot of the chill).
Place lamb (along with root vegetables, if desired) in roasting pan and roast at 450F for approximately 15 minutes, then reduce oven heat to 350F and continue roasting until an instant-read thermometer reads 145-150F for medium rare, or 160F+ for well done, allowing that the meat will raise about 5 degrees as it rests after cooking.
Start checking your meat temp with the instant read thermometer at about 20-25 minutes per pound.
Allow the roast to rest undisturbed for about 25 minutes before carving.
Reduce any drippings, if desired, to serve with the lamb.