Boil a pan of water and add the cauliflower florets. Cook for about 10 minutes or until tender and then drain.
Fry the bacon lardons until coloured, remove from the pan and set aside but leave the fat in the pan. Add the mushrooms and fry for 2 -3 minutes then set aside.
Melt the butter in a saucepan and add the flour to make a roux. Stir in the mustard powder, grated cheese and double cream. The mixture should be thick and creamy once the cheese has melted. Season to taste, then add the bacon and mushrooms.
Preheat the oven to 180°C/350°F/Gas 4. Place the cauliflower in an ovenproof casserole dish and pour over the sauce. Sprinkle a pinch of freshly ground nutmeg over the top.
Mix the breadcrumbs with the parmesan/monchego cheese and sprinkle over the mixture. Cook for 15 minutes or until the topping is golden-brown and bubbling.
Spoon the cauliflower cheese onto plates and serve with a crisp salad and crusty bread.