Cut the turkey bacon into pieces using kitchen shears for ease, then add it to a soup pot. Cook the bacon over medium-high heat until it becomes crispy, about 10 minutes.
While the bacon cooks, prepare the onion and potatoes. You can peel the potatoes or simply scrub them if you prefer to leave the skin on.
Once the bacon is done, remove it from the pot and place it on a paper towel-lined plate to drain. Leave approximately two tablespoons of bacon fat in the pot. Discard the remaining fat or save it for another recipe.
Add the chopped onion to the pot and sauté for about 5 minutes.
Sprinkle in the flour and cook, stirring, for about one minute.
Slowly whisk in the milk until the flour is fully dissolved.
Add the chicken broth, minced garlic, and diced potatoes. Turn the heat up to high and scrape any browned bits from the bottom of the pot. When the soup starts to boil, reduce the heat to low and cover the pot with the lid slightly ajar. Let the soup simmer gently for 20 minutes.
While the soup simmers, prepare the cheddar cheese, scallions, and sour cream.
The potatoes should be very tender and starting to fall apart. You can mash them a bit directly in the pot using a potato masher, or leave them as they are if you prefer.
Stir in the sour cream along with most of the cheddar cheese and cooked bacon, reserving some for garnish. Season the soup with salt and pepper to taste.
Serve the soup topped with the remaining cheddar, bacon, and chopped scallions.