Steam cauliflower for 10-15 minutes or until just tender. Use 1 tbsp butter to grease oven-proof baking dish and place cauliflower in it. Set aside.
Add remaining butter and flour to a saucepan over medium heat and whisk constantly until blended. Pour in milk and whisk continuously until it is fully incorporated then let it simmer for another 2-3 minutes.
Stir in three-quarters of the grated cheese, mustard, nutmeg, thyme, salt and pepper and cook until smooth. Pour evenly over the cauliflower.
Sprinkle remaining cheese and breadcrumbs evenly over the top.
Bake at 450° for about 20-25 minutes or until golden brown and bubbling. Serve immediately.