Melt 2 tablespoons Becel® Buttery Taste margarine in 2-quart saucepan over medium heat and cook flour, stirring frequently, 1 minute. Gradually whisk in milk. Bring to a boil and cook until thickened, about 1 minute. Stir in cheese and mustard until cheese is melted.
Add broccoli and cook over medium-low heat, stirring frequently, until heated through. Turn into greased 1-1/2-quart shallow casserole.
Combine remaining 2 tablespoons Becel® Buttery Taste margarine, melted with bread crumbs, then sprinkle over broccoli mixture. Broil until golden, about 1 minute.
