Preheat the oven to 180°C and season the lamb with salt and pepper.
Heat a casserole dish and sear the lamb in oil and butter until browned.
Sear the kidneys in the same pan, then cook onions and rosemary for 10 minutes.
Stir in flour, reduce the wine by half, and add stock and Worcestershire sauce.
Return lamb to the pan with herbs, then layer potato slices on top.
Season potatoes, brush with melted butter, and bake covered for 1.5 hours.
Uncover and bake at 200°C for 25 minutes until the potatoes are crisp.
Boil cauliflower florets for 2 minutes, drain, and place in a buttered dish.
Make a roux with butter and flour, kemudian whisk in milk until smooth.
Stir in egg yolks, mustard, and half the cheese, then pour over cauliflower.
Top with remaining cheese and grill until golden-brown.
Serve the hotpot alongside a portion of the cauliflower cheese.