MELT butter in large saucepan or Dutch oven on medium heat. Add yellow onions and celery; cook and stir 5 min. or until crisp-tender. Add next 4 ingredients; bring just to boil, stirring constantly. Slightly crush potatoes with back of spoon. Simmer on medium-low heat 5 min., stirring frequently.
RESERVE 1 Tbsp. green onions and 2 Tbsp. each bacon and cheese. Add remaining green onions, bacon and cheese to soup; cook 5 min. or until cheese is melted, stirring constantly.
SERVE topped with reserved green onions, bacon, cheese and sour cream.