Preheat oven to 350°F (180°C). Spray 2L baking dish with no-stick cooking spray; set aside.
Melt margarine in large saucepot over medium-high heat and cook onion, stirring frequently, until tender, about 4 minutes. 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 250 mL cheese; let stand 1 minute. Stir in broccoli and mustard. Pour into prepared dish.
Bake 20 minutes. Top with remaining 125 mL cheese and bake an additional 5 minutes or until cheese is melted. Let stand 10 minutes before serving.