PIERCE potatoes. Microwave on HIGH 5 min., turning over after 2-1/2 min. Mix broth and milk in large microwaveable bowl. Add potatoes. Microwave on HIGH 6 min., stirring after 3 min. Mash potatoes lightly.
RESERVE 2 Tbsp. each bacon and cheese, and 1 Tbsp. onions; set aside. Stir remaining bacon, cheese and onions into soup.
SERVE bowls of soup topped with sour cream, and reserved bacon, cheese and onions