Preheat the oven to 180C/160C Fan/Gas 4. Season the lamb with sea salt and pepper.
Heat a large ovenproof casserole until hot, then add the oil and butter and sear the lamb until browned all over. You’ll need to do this in batches.
Set the lamb aside, then briefly sear the lambs’ kidneys in the same pan. Add the onions and rosemary adding a little more butter to the pan if necessary, and cook for about 10 minutes until they turn brown at the edges.
Stir in the flour and cook for a minute, add the red wine and reduce by half then add the stock and Worcestershire sauce. Whisk until the flour and liquid are smoothly blended. Season with sea salt and pepper and bring to simmering point. Return the lamb to the pan then add the bay leaf and thyme.
Arrange the potato slices on top of the meat, overlapping each other. Season the potatoes with sea salt and pepper then brush with a little melted butter.
Cover with a tight fitting lid and bake for 1½ hours. Remove the lid and turn the oven up to 200C/180C Fan/Gas 6. Cook for a further 20-30 minutes until the potatoes are golden-brown and crisp.
Meanwhile, for the cauliflower cheese, preheat the gill to high.
Bring a large pan of salted water to the boil, then add the cauliflower florets and cook for 2 minutes. Drain and place the cauliflower in a buttered ovenproof dish.
Place the butter and flour in a saucepan and heat to melt the butter. Cook, stirring for 1-2 minutes until lightly golden-brown. Remove the pan from the heat and gradually whisk in the milk to form a smooth sauce. Return the sauce to the heat and simmer for 2-3 minutes. Remove from the heat and beat in the egg yolks and mustard and half the cheese. Season with salt and pepper then pour over the cauliflower and top with the remaining cheese. Place under the grill and cook until golden-brown.
Place spoonfuls of the hotpot and cauliflower cheese on warmed plates and serve.