Heat oven to 160C/fan 140C/gas 3.Heat the oil in a large heavy based pan. Sizzle the bacon for 4 mins. Then add the lamb and cook for 6 mins until brown. Remove the meats with a slotted spoon and set aside.
Add the onions, carrots and herbs to the pan, and cook for about 5 mins until softened. Return the meat to the pan, stir in the pearl barley, pour over the stock, then bring to a simmer.
Sit the chunks of potato on top of the stew, cover, then braise in the oven, undisturbed, for about 1½ hrs until the potatoes are soft and the meat is tender.
Remove from the oven, dot the potatoes with butter, scatter with the spring onions and serve scooped straight from the dish.