Melt butter in medium saucepan on medium-high heat. Add onions; cook 3 to 5 minutes or until crisp-tender, stirring frequently. Whisk in flour until blended.
Add broth and milk; cook on medium-high heat 2 minutes or until heated through, stirring frequently. Add Velveeta and broccoli; cook and stir 5 minutes or until Velveeta is melted and broccoli is heated through.