Cut the carrots in half lengthways and stud the onion with 2 of the cloves. Place the ham hocks in a deep saucepan, add the onion, carrots, celery and herbs, cover with cold water and add a pinch of salt. Bring slowly to the boil over a medium-low heat, then gently simmer for 2 hours. Turn off the heat and allow to cool slightly.
Preheat the oven to 200°C. Carefully remove the ham hocks from the liquid and place them in a roasting tin. Using a sharp knife, remove the skin and trim off any excess fat to reveal the pink meat underneath, then insert cloves at regular intervals over each hock. Return the hocks to the oven for 10 minutes to dry the surface of the meat.
Meanwhile, gently warm the black treacle in a saucepan. After 10 minutes, remove the hocks from the oven, spoon over the treacle and return to the oven for a further 20–30 minutes, basting occasionally.
Separate out the leaves of the lettuce. Once the hocks are shiny and glazed, remove from the oven and serve with the lettuce leaves and a dollop of wholegrain mustard.