MICROWAVE potatoes in large microwaveable bowl on HIGH 5 min., stirring after 2-1/2 min. Stir in broth and milk. Microwave 10 min., stirring after 5 min. Carefully smash potatoes with potato masher.
RESERVE 2 Tbsp. each bacon and cheese, and 1 Tbsp. onions for topping. Stir remaining bacon, cheese and onions into soup.
SERVE topped with sour cream and reserved ingredients.