Cut the cauliflower into big florets and steam until just fork tender.
Preheat the oven to 400 degrees F. and butter a 9x11-inch casserole dish.
In a large heavy-bottomed saucepan melt the butter over medium-low heat. Stir in the flour and cook 2-3 minutes, stirring often.
Slowly whisk in the milk and turn the heat up to medium-high. Bring to a simmer and stir 2 -3 minutes.
Whisk in the mustard. Turn the heat down to low.
Add the cheese a handful at a time, stirring until completely melted and smooth. Remove from heat.
Place all the cauliflower in the baking pan. Pour the cheese sauce over it.
Toss the bread crumbs and Parmesan together. Sprinkle it over the cauliflower.
Bake 25 minutes until bubbly and golden. Let set 5 - 10 minutes before serving.