Add diced potatoes, diced onion, minced garlic, cooked bacon, dried parsley, and chicken broth to the slow cooker. Season with salt and pepper.
Stir and cook for 6-8 hours on low or on high for 3-4 hours.
Once the soup has cooked, add butter to a medium saucepan over medium heat and melt. Whisk in the flour until completely combined and gradually add in the evaporated milk or heavy cream, depending on which one you are using.
Whisk well the flour mixture, until smooth. With the heat on the lowest stove setting, let the mixture cook until it starts to simmer and is getting thicker, stir occasionally.
Immediately add the milk-flour mixture to the slow cooker and stir to combine.
Or make the Cornstarch Mixture:
If you don't want to make the Roux Mixture, you can make a Cornstarch Mixture to thicken the soup. Just make sure you choose one and don't make both.
Combine cornstarch and evaporated milk in a medium bowl. Whisk until cornstarch is fully dissolved and the mixture is smooth with no clumps. Immediately add the milk-cornstarch mixture to the slow cooker and stir to combine.
After the Thickening Mixture was added:
Using a potato masher, mash about 3/4 of the potatoes. If you have time, let the soup cook for another 30 minutes for it to thicken.
Stir in the shredded cheddar cheese and sour cream. Stir well, until fully combined and creamy. Season with salt and pepper.
If the soup is too thick, add more WARM chicken broth or water, make sure you first heat it up in the microwave until it's hot. Taste and adjust for salt and pepper.
Cover and continue cooking the soup on low for 30 minutes or on high for 15 minutes. If you added more liquid, make sure to let the soup simmer for a few minutes.
Serve warm, garnished with bacon, cheese, green onions, and sour cream.
The soup can be refrigerated for up to 3 days.
The soup will thicken more once it cools. When heating it up, add more water, broth, or milk to dilute it. Taste and adjust for salt and pepper and if you added the extra liquid, bring it to a light simmer for a few minutes.