Preheat the oven to 180°C. Layer the lamb cutlets, onion and carrot in a large casserole, sprinkling each layer with salt, pepper and chopped parsley. Put the oysters (if using) on top and spoon over their juices. Arrange the potatoes, overlapping in a double layer, on top of the oysters. Pour over the stock and sprinkle with salt and pepper and the thyme.
Cover the casserole and cook the hotpot in the centre of the oven for 2 hours, then increase the temperature to 230°C. Remove the lid from the casserole, dot the potatoes with the butter and continue to cook, uncovered, for a further 30 minutes, or until the potatoes are golden brown. Serve immediately with steamed cabbage or spinach, if desired.