Boil the cauliflower florets in a large pan for 10 minutes until just tender, then drain.
Fry the bacon strips in a dry pan until colored and set aside while retaining the fat.
Fry the mushrooms in the bacon fat for 3 minutes until golden-brown and set aside.
Melt butter in a saucepan, beat in flour, then whisk in milk, mustard, cheese, and cream until smooth.
Season the sauce and fold in the cooked bacon and mushrooms.
Preheat the oven to 180°C.
Place cauliflower in a casserole dish, pour over the sauce, and sprinkle with nutmeg.
Mix breadcrumbs with Parmesan, sprinkle over the top, and bake for 15 minutes.
Serve the cauliflower cheese with green salad and crusty bread.