Preheat oven to 450 degrees F. Remove lamb from marinade, letting excess drip back into bag.
Place lamb on a plate, and season generously with smashed garlic, salt and pepper.
Pour marinade into roasting pan and add assorted vegetables. Toss with olive oil, and season with salt and pepper.
Arrange the seasoned leg of lamb on top of the vegetables.
Roast 15 minutes, then cover with foil and reduce oven temperature to 300 and cook until medium-rare. About 1 1/2 hour. (An instant-read thermometer inserted in the center should read 125 when inserted in the thickest part of the lamb.)
Allow leg of lamb to rest for 20 minutes before slicing. Toss vegetables with pan juices and serve.