cook the bacon: place the chopped bacon in a large dutch oven or heavy-bottomed pot over medium heat. cook, stirring occasionally, until crisp, about 8-10 minutes. use a slotted spoon to transfer the bacon to a paper towel-lined plate. reserve about 2 tablespoons of the bacon fat in the pot.

sauté aromatics: add the diced onion and celery to the bacon fat in the pot. cook, stirring, until softened, about 5-6 minutes. add the minced garlic and cook for 1 more minute until fragrant.

make the roux: melt the 4 tablespoons of butter into the pot. once melted, whisk in the ¼ cup of flour and cook for 1-2 minutes, stirring constantly. this cooks out the raw flour taste.

build the soup: slowly pour in the chicken broth while whisking constantly to prevent lumps. once smooth, whisk in the milk. add the diced russet potatoes and season with salt and pepper.

simmer: bring the soup to a simmer, then reduce the heat to medium-low. cover and cook for 15-20 minutes, or until the potatoes are very tender when pierced with a fork.

thicken and mash: reduce the heat to low. use a potato masher to mash about half of the potatoes directly in the pot. this will thicken the soup while still leaving plenty of hearty potato chunks. for a smoother soup, you can use an immersion blender for just a few quick pulses. do not over-blend.

load it up (off heat): remove the pot from the heat. stir in the heavy cream and sour cream until combined. slowly add the shredded cheddar cheese, stirring until it is completely melted and the soup is smooth.

serve: stir in half of the reserved crispy bacon. taste and adjust seasoning with more salt and pepper if needed. ladle the soup into bowls and garnish with the remaining bacon, extra cheddar cheese, and fresh green onions or chives.