Melt butter in a large saucepan on medium heat. Add onion; cook and stir until onion is tender, about 5 minutes. Add flour; cook 1 minute or until bubbly, stirring constantly.

Stir in milk. Bring to boil. Reduce heat to medium-low; simmer for 1 minute.

Add remaining ingredients; Velveeta cheese, broccoli, and pepper. Cook until cheese is melted and soup is heated through, stirring occasionally.