1. Preheat the oven to 375°F. Butter an 11 x 7-inch glass baking dish with 1 Tbsp. butter.
2. Pour 3 cups of water into the bowl. Place cauliflower florets in the steam basket and place the basket in the Prep&Cook bowl. Launch the P1 steam program at 210°F for 15 minutes. At the end of the program, place cauliflower in the prepared dish. Empty the water from the Prep&Cook bowl.
3. Place remaining butter in the bowl. Heat at 110°F for 2 minutes or until melted. Remove 2 Tbsp. to a separate bowl and mix with breadcrumbs; set aside.
4. Place flour, milk, ground mustard, salt and pepper in the bowl fitted with the whisk. Mix at speed 6 for 30 seconds. Launch the sauce program at speed 4 at 190°F for 8 minutes.
5. At the end of the program, add the cheese. Mix at speed 6 for 1 minute. Pour over the cauliflower and mix slightly. Top with the breadcrumb mixture.
6. Bake for 20 minutes or until thoroughly heated and golden brown on top.