In a Dutch oven or large soup pot, cook the bacon over medium heat until all the fat has rendered.
Using a slotted spoon, transfer the cooked bacon to a paper towel-lined plate and drain off all but 1/4 cup of the drippings.
Reduce the heat to medium-low and then add the celery and onions to the pan and cook, stirring occasionally, until the onions are translucent.
Add the garlic and continue cooking for 1 to 2 minutes.
Increase the heat to medium and then add the cubed potatoes, stirring to combine. Cook the potatoes for 5 minutes, stirring occasionally, then add the chicken stock. (The chicken stock should just cover the potatoes, so depending on your pot, you may have to add more stock.) Bring the soup to a simmer then cover the pot and cook the soup for 10 minutes, or until the potatoes are fork-tender.
In a separate medium saucepan, melt the butter over medium heat. Once the butter is melted, whisk in the flour to make a roux. Stir the roux constantly for 1 to 2 minutes until it's pale golden then whisk in the milk.
Bring the mixture to a boil and cook it, stirring constantly, until it's thickened slightly. (It should be the consistency of gravy.) Whisk the milk mixture into the potato mixture and cook the soup for 5 minutes over medium-low heat.
Remove the soup from the heat. Transfer 1/3 of the soup to a medium bowl. Using a hand blender, purée the soup that's remaining in the pot until it's smooth.
Return the reserved 1/3 of the soup to the pot, stirring to combine.
Stir in the heavy cream and season the soup with salt and pepper. When ready to serve, top each bowl of soup with the cooked bacon, shredded cheddar cheese, sour cream and scallions.
If you don't have a hand blender, you can purée the soup in batches in a blender. Leave 1/3 of the soup in the pot and purée the rest in the blender then continue with the recipe as directed.
I prefer seasoning my soups at the end to avoid over-salting them. That being said, potatoes need a hearty amount of salt, so don't be afraid to start with a teaspoon of salt and at least a few turns of fresh black pepper and work your way up based on your personal preferences.