Preheat oven to 350F.
Boil cauliflower in a large pot of salted water until tender-crisp, about 5-6 min. Drain and set aside.
Melt ¼ cup butter in skillet over medium. Add garlic and cook 2 min. Whisk in flour and mustard powder and cook 1 min. Whisk in milk and cook until thickened, about 5 min. Stir in cheese and salt and cook until melted, about 1 min. Season with fresh pepper.
Add cauliflower and toss to coat. Pour into 2-L baking dish and bake, covered, until the cheese mixture is bubbling at the sides of the dish, about 40 minutes.
Stir remaining butter with bread crumbs and Parmesan. Sprinkle mixture over cauliflower and broil until golden, checking often, about 2 min.
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Last updated: October 27th, 2020
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