Spring may have sprung, but it's still freezing in Melbourne! I think this is essentially a Lancashire Hotpot, and very tasty.
Preheat oven to 170 C. Grease an ovenproof casserole dish.
Place half the margarine in a large frypan over low heat and cook leek and onion for 5 minutes or until softened. Transfer to a plate and set aside.
Add oil to pan, increase heat to high. When oil is hot, add lamb and brown in batches, then transfer to a separate plate. Add garlic and barley to pan, season with salt and pepper and cook for 1 minute, then return lamb to pan and stir to combine. Place half the potatoes in base of casserole. Layer meat over top followed by leek mixture. Use back of a spoon to flatten contents of casserole, then overlap remaining potatoes on top. Pour in stock and sprinkle with thyme. Cover with a lid or a layer of foil and bake in oven for 2 hours. Brush potatoes with olive oil or melted margarine. Return to oven, uncovered, for 30 minutes or until potatoes are golden.
Serve from dish, sprinkled with parsley.