Preheat the oven to 450°. To prepare the lamb, combine the first 8 ingredients (through garlic) in a large bowl; add 4 tablespoons of the oil, 1/2 teaspoon salt, and 1/4 teaspoon black pepper. Toss well to combine. Place the vegetable mixture in the center of a large, deep roasting pan. Rub the remaining 2 tablespoons oil over the lamb, and sprinkle with the remaining 1/2 teaspoon salt and 1/4 teaspoon black pepper.
Place the lamb on top of the vegetable mixture. Insert a meat thermometer into the thickest portion of the lamb. Bake at 450° for 50 minutes. Reduce the oven temperature to 350° (do not remove the lamb from the oven). Bake 20 minutes more, or until the thermometer registers 150° (for medium-rare). Roast to desired degree of doneness, stirring the vegetables every 15 minutes. Place the roast on a heated platter, and cover with foil. Let stand 15 minutes before carving.
Drain the vegetables through a fine sieve into a bowl (reserve the liquid for sauce). Place the vegetables in a bowl, and stir in the parsley and vinegar.
To prepare the gremolata, combine the ingredients in a small bowl, and toss to combine. Serve a small spoonful with the lamb.