Pierce potatoes; microwave on HIGH 5 minutes, turning over after 2 1/2 minutes. Cut potatoes into chunks. Combine potatoes, broth, and milk in a large microwaveable bowl. Microwave on HIGH 6 minutes, stirring after 3 minutes. Slightly crush potatoes with a potato masher.
Reserve 2 tablespoons each of the bacon and cheese and 1 tablespoon of the onions for topping; set aside. Add remain bacon, cheese, and onions to soup.
Serve topped with reserved bacon, cheese, and onions.