Combine carrots, celery and green onions in water; cook until tender. Set aside. Saute onion in butter until limp; stir in the flour and blend well. Do not brown. Combine milk and broth; bring to a boil. Whisk in onion mixture. Add cheese dip, cayenne, salt and pepper, and mustard. Slowly stir in vegetables and water they were cooked in. Bring just to a boil; serve immediately.