Preheat the oven to 180°C / fan oven 170°C / Gas 4. Heat the oil over a high heat in a large flameproof lidded casserole if you have one, or a large deep frying pan. Add the lamb and brown quickly all over (about 5 minutes). Do this in batches unless your pan is big.
Add the onions to the pan and fry gently for 7 to 10 minutes until they are softened and lightly browned. Stir in the garlic.
Add the tomatoes, lemon, oregano, stock, salt and pepper, and bring to the boil. If using a flameproof casserole, simply cover; if not, transfer from the frying pan to a suitable ovenproof dish with a tightly fitting lid. Cover and cook in the oven for 1 ¼ hours. Add the potatoes, cover the dish again and cook for 45 minutes more until the lamb and potatoes are tender.
Remove from the oven and crumble over the feta. Cover and leave to stand for 10 minutes so the cheese melts. Scatter over the parsley and serve in soup bowls with thick slices of crusty bread to mop up the juices.
