Preheat oven to 350°. Spray 2-quart baking dish with nonstick cooking spray; set aside.
Melt Spread in 4-quart saucepot over medium-high heat and cook onion, stirring frequently, 4 minutes or until tender. Reduce heat to medium, then stir in flour and cook, stirring constantly, 2 minutes. Gradually stir in milk. Bring to a boil over high heat. Stir in Knorr® Homestyle Stock - Chicken and boil until Stock is melted and sauce is thickened, about 2 minutes. Remove from heat, then stir in 1 cup cheese; let stand 1 minute. Stir in broccoli and mustard. Pour into prepared dish.
Bake 20 minutes. Top with remaining 1/2 cup cheese and bake an additional 5 minutes or until cheese is melted. Let stand 10 minutes before serving.