1.Preheat grill. Oil a 2-litre (8-cup) ovenproof dish.
2.Cook the pasta in a large saucepan of boiling water until al dente. Add the cauliflower to the pan during the last few minutes. Drain.
3.Meanwhile, add the bacon to a medium saucepan, cook, stirring, over medium heat until browned lightly. Add the butter to the pan then the flour; cook, stirring, until bubbling. Gradually stir in the milk; cook, stirring, until mixture boils and thickens. Remove from heat; stir in half the cheeses. Season to taste.
4.Combine pasta mixture and cheese sauce in a large bowl. Spoon mixture into prepared dish, top with combined breadcrumbs and remaining cheeses.
5.Place dish under hot grill for 10 minutes or until top is golden.
Suitable to freeze.
If you want to prep ahead of time, bake instead of using the grill. Bake at 200C (180C fan forced) for 30 minutes or until browned and heated through.