Dice the lamb into bite sized pieces. Mix together the salt and the flour and the coat the lamb pieces in this mixture. Peel and dice the carrots. Cut the celery into small pieces.
Heat a heavy bottomed frying pan and add the oil. Add the lamb to the pan and colour on all sides. Add the carrots and celery to the pan and cook for a few minutes before adding the stock, thyme and bay. Simmer for 10 minutes until the liquid has thickened. Pour the mixture into a casserole dish, leaving enough space to add the potato topping.
Preheat the oven to 180'C.
Peel and slice the onions and the potatoes and then cut into fine discs using a mandolin or a sharp knife. Fry the onions in 3/4s of the butter until they are soft and golden. Spread the onions on top of the meat mixture.
Melt the rest of the butter and then use it to coat the potatoes. Season with salt and pepper and then layer the potatoes neatly on top of the onions to create a pattern resembling fish scales. Transfer to the oven and cook for 45 minutes to an hour. The potatoes should be nice and golden brown on top.
«Use cheaper cuts of lamb, such as shoulder or neck and cook longer and slower for a more tender and flavoursome dish. If the potato topping starts to brown too quickly, cover the casserole dish with a lid or aluminium foil.»