Press sumac and rosemary over entire piece of lamb to form a crust.
Drizzle olive oil over crust and let it marinade for several hours. If possible, leave overnight.
Preheat oven to 425°F (220°C).
Place lamb on a shallow pan and roast in oven for 15 minutes.
Reduce heat to 350°F (180°C) and continue roasting for another 1½ hours or until thermometer inserted into the thickest portion registers 160°F (17°C) to 165°F (74°C) for medium doneness.
Remove and cover with foil. Let it rest for 15 to 20 minutes before carving.