Cook Cauliflower in boiling salted water for 5 mins or until just tender, drain well & arrange in baking dish.
Heat butter in a saucepan over medium heat and cook onion and half the bacon for 4 mins or until softened. Remove pan from heat, add flour and stir with a wooden spoon until combined. Return to heat & cook gently for 2 minutes.
Gradually add milk, stirring continuously and bring to a boil. Add mustard, nutmeg, 1 cup cheese and season with salt & pepper. Stir until cheese has melted.
Pour sauce over cauliflower & sprinkle with breadcrumbs and remaining cheese. Bake for 15 mins or until golden.