Place first 10 ingredients in a food processor; pulse until finely chopped. Place onion mixture and lamb in a large zip-top plastic bag. Seal bag; rub onion mixture into lamb. Marinate in refrigerator for 4 hours to overnight, turning occasionally.
Preheat oven to 450°.
Remove lamb from bag; discard marinade. Place lamb on a broiler pan coated with cooking spray. Bake at 450° for 15 minutes. Decrease oven temperature to 350° (do not remove lamb from oven). Bake at 350° for 1 hour and 20 minutes or until a thermometer inserted in thickest portion of the lamb registers 145° (medium-rare). Place lamb on a cutting board. Cover loosely with foil; let stand for 15 minutes.
I made this with roasted pattypan squash and the lemon orzo with pine nuts. Used local grass-fed lamb shoulder roast. I roasted my 2.25 lb. piece of meat for 15 min at the 450, then cut to 350 and roasted 25 min. per pound more. Turned out just right! The meat itself was delicious, although I would probably look for a spicier or more dramatic flavor in the marinade. This was subtle and lacked a certain zip I prefer in foods.
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