1. Preheat the oven to 400°F.
2. Put the lamb in a large roasting pan. With the tip of a sharp knife, make 12 shallow slits in the top surface of the meat. Insert the garlic slivers.
3. In a medium bowl, combine the pistachios, panko, 2 tablespoons of the oil, and the mustard. Stir until blended. Stir in the rosemary, zest, salt, and pepper until well mixed.
4. Firmly pat the nut mixture over the top of the meat, being sure to cover the garlic-filled slits. Loosely tent the crust with a 12-inch square of aluminum foil and transfer to the oven. Immediately reduce the oven temperature to 325°F and roast for 1 1/2 hours.
5. In a medium bowl, toss the potatoes, carrots, and onion quarters with the remaining 4 tablespoons oil. Season with salt and pepper to taste. Remove the foil tent and scatter the vegetables around the roast. Continue roasting 1 1/2 hours longer, or until an instant-read thermometer inserted into the center of the roast and not touching the bone registers 145°F for medium-rare. If the crust begins to darken near the end of roasting, replace the foil tent.
6. Transfer the roast to a platter and let rest for about 5 minutes.
7. Arrange the vegetables around the roast and serve.