Broccoli & Cheese Casserole

WHAT YOU'LL NEED

• 2 Tbsp. (30 mL) margarine
• 1 small onion
• 2 Tbsp. (30 mL) all-purpose flour
• 1 1/2 cups (375 mL) 2% milk
• 1 pot Knorr® Homestyle Stock Chicken
• 1 1/2 cups (375 mL) shredded cheddar cheese, divided
• 8 cups (2 L) broccoli florets or cauliflower florets, blanched just until tender
• 1 Tbsp. (15 mL) Dijon mustard