In a 6-qt. stockpot, heat butter over medium heat. Add celery, carrots, green pepper and onion; cook and stir 3-4 minutes or until onion is tender. Stir in flour and seasonings until blended., Gradually stir in milk. Add broth, potatoes and corn; bring to a boil. Reduce heat; simmer, covered, 15-20 minutes or until potatoes are tender., Stir in cheese until melted; remove from heat. Puree soup using an immersion blender. Or, cool slightly and puree in batches in a blender; return to pot and heat through. Stir in parsley.