Have your butcher trim the pelvic bone and as much fat as possible from a whole 6-pound leg of lamb to get the 4 ½ pounds you need. Roast lamb on a rack so that any fat drains away from the meat and vegetables.

1. Move the oven rack to its lowermost position. Preheat oven to 400°F.

2. Add the garlic, salt, pepper, and rosemary to a small bowl; mix well. Reserve 1 ½ teaspoons of the mixture. Use a sharp knife to make 1" deep incisions in the top of the leg and around the meatier parts of the leg. Rub what remains of the herb mixture into those incisions and onto all sides of the meat. Sprinkle additional salt and pepper over the outside of the meat, if desired. Set the roast topside up on a rack in a large roasting pan.

3. Prepare two large sheets of double thickness of heavy-duty aluminum foil, one large enough to hold all of the potatoes and the other large enough to hold the carrots. Add the oil and remaining garlic-herb mixture to a small bowl; mix well. Place the potatoes in the center of one of the foil pieces; drizzle with ⅓ of the oil mixture, rubbing it into the potato skins.

4. Bring the sides of the foil up and over the potatoes and crimp it on top to form a packet; put at one end of the roasting pan. Place the carrots on the other foil piece; drizzle with ⅓ of the oil mixture. Bring the sides of the foil up and over the carrots and crimp it on top to form a packet; put at the other end of the roasting pan. Put the roasting pan in the oven for 30 minutes.

5. Remove the roasting pan from the oven; turn the roast over. Open each of the foil packets. Form one or two bowls out of doubled sheets of heavy-duty aluminum foil; add the asparagus and drizzle with the remaining oil mixture. Position in the roasting pan. Return the roast to the oven for 45 minutes, or until the thickest part of the leg reaches an internal temperature of 130°F. Remove from the oven and transfer the roast and all vegetables to a large serving platter; tent with foil and let rest for 30 minutes before carving the meat.

6. To make a meat sauce, drain as much of the fat as possible from the roasting pan, then use paper towels to blot any fat that remains in the pan. Put the roasting pan on the stovetop over medium heat. Bring to a simmer, using a spatula to scrape up any browned bits from the bottom of the pan. Simmer for 15 minutes, stirring occasionally, then whisk in mint or redcurrant jelly to taste, if desired. Serve the sauce as is or strained over the meat.