Preheat the grill to its highest setting.
Boil cauliflower florets in salted water for 6 minutes, then refresh in iced water.
Melt 50g of butter and whisk in flour to form a smooth paste.
Gradually whisk in milk, then stir in mustard and 150g of grated cheese.
Place drained cauliflower in an ovenproof dish, cover with sauce, and top with remaining cheese.
Grill for 5 minutes until the cheese is golden and bubbling.
Fry brioche crumbs in the remaining butter until crisp and golden.
Dry-fry pancetta until crisp, then glaze with maple syrup.
Top the cauliflower cheese with crumbs and pancetta rashers before serving.