• Preheat oven to 200°. Purée blackberries in a blender or food processor until smooth. Press through a sieve to remove seeds. Discard solids. Combine blackberry purée, Dijon mustard, vinegar, 6 cloves garlic, rosemary, thyme, salt and pepper in a medium bowl. Set aside.
• Pierce meat using meat fork every 1 inch. Rub half the blackberry herb mixture over the lamb, coating the outside completely. Sprinkle remaining 24 peeled halved garlic cloves in a large roasting pan. Place lamb in roasting pan. Let stand 20 minutes.
• Roast 30 minutes. Reduce oven temperature to 180° and continue to roast 1 hour longer until meat thermometer reads 145°F for medium-rare. Remove lamb from pan and let rest for 10 to 15 minutes before carving.

Blackberry Herb Sauce

• Pour off all but 1 tablespoon drippings from roasting pan. Add 250ml chicken stock to roasting pan and stir with a wooden spoon to loosen and dissolve any browned bits. Pour stock mixture into a blender or food processor and purée until smooth.
• Combine garlic broth mixture, remaining chicken stock, remaining blackberry purée and cornflour in a small saucepan. Bring to a boil over medium heat, stirring constantly; boil 1 minute. Stir in blackberries. Serve over sliced lamb.