Finely chop walnuts and garlic in processor. Add 2 cups mint leaves and basil and chop finely. Add oil and vinegar and blend until pesto is smooth. Cut excess fat from lamb, leaving thin layer. Set lamb in large roasting pan. Make several slits in lamb with tip of small sharp knife. Reserve 1/2 cup pesto for vegetables. Spoon some pesto into each slit; rub remainder into lamb. Sprinkle lamb generously with salt and pepper. Let stand 1 hour at room temperature or cover and chill overnight. Preheat oven to 350F. Roast lamb 45 minutes. Add potatoes to pan and turn to coat in pan juices. Roast 30 minutes. Add carrots and reserved 1/2 cup pesto and turn to coat carrots and potatoes in pesto and pan juices. Sprinkle with salt and pepper. Continue roasting until thermometer inserted into thickest part of lamb registers 140F. for medium-rare, about 45 minutes longer. Remove from oven and let stand 15 minutes. Carve lamb into thin slices. Arrange on plates with potatoes and carrots. Garnish with fresh mint leaves. Serves 6. Bon Appetit April 1992