1. Put the lamb in a large saucepan and add the stock. Quarter one of the onions, leaving the skin on, and add to the pan with the carrots. Bring to a gentle simmer, cover with a lid and cook on the lowest heat for 1½ hours. Lift out the meat onto a board and strain the stock into a large bowl or jug. Clean the pan.
2. Roughly chop the remaining onion (peeled) and the leeks, keeping the green tops separate. Melt the butter in the cleaned pan and fry the onion and white parts of the leeks for 5 minutes, until soft.
3. Cut the meat away from the bone, discarding any very fatty parts. Cut into bite-size pieces and return to the pan with the potatoes and farro. Skim the fat off the stock with a large spoon and return the stock to the pan.
4. Bring to a gentle simmer and cook for 15 minutes until the potatoes and farro are just tender. Remove from the heat and stir in the green leek ends, the balsamic glaze and mint. Transfer to an ovenproof serving dish. Leave to cool.