First trim the lamb of any excess fat and wipe with kitchen paper. In a large frying pan, heat the oil and 1/4 oz (5 g) butter until it is very hot, then brown the pieces of lamb two or three at a time until they all have a good brown crust. As they cook, remove them to a wide casserole. Brown the pieces of kidney too, and tuck these in among the meat.

Next, fry the onions – add 1/4 oz (5 g) butter to the pan if you need any extra fat – cooking them for about 10 minutes till they turn brown at the edges. Now stir in the flour to soak up the juices, then gradually add the hot water and Worcestershire sauce, stirring or whisking until flour and liquid are smoothly blended. Season with salt and pepper and bring it up to simmering point, then pour it over the meat in the casserole. Add the bay leaf and thyme, then arrange the potato slices on top, in an overlapping pattern like slates on a roof. Season the potatoes and add a few flecks of butter here and there. Cover with a tight-fitting lid and cook near the top of the oven for 11/2 hours, then remove the lid and cook for a further 50 minutes.

I sometimes finish off the hotpot under the grill. If you brush the potatoes with a little more butter and place the casserole under a hot grill they crisp up and brown beautifully. Alternatively, if you think they're not browning enough during cooking, you can turn the heat in the oven right up during the last 15 minutes. Remove the bay leaf and thyme sprigs before serving.