Cut bacon into small pieces using kitchen shears. Cook in a large soup pot over medium-high heat until crispy, 12-15 minutes for thick-cut or 8-10 minutes for regular. Transfer to paper towel-lined plate, leaving 2 tablespoons bacon fat in pot.

Add onion to bacon fat and cook for 5 minutes until softened and lightly caramelized. Stir in flour and cook for 1 minute, stirring constantly, until mixture smells nutty.

Slowly whisk in milk, starting with a splash and whisking until smooth before adding more. This prevents lumps and ensures silky texture.

Add chicken broth, garlic, and diced potatoes. Increase heat until small bubbles form around edges of pot, then reduce heat and cover with lid slightly ajar.

Simmer gently for 20 minutes until potatoes are fall-apart tender when pierced with a fork.

Using a potato masher, mash about half the potatoes directly in the pot to create desired texture - creamy with some chunks remaining.

Remove from heat. Stir in sour cream, most of the cheese, and most of the bacon, reserving some for garnish. Season generously with salt and pepper to taste.

Serve hot, topped with remaining bacon, cheese, and chopped scallions. Add chicken broth if soup becomes too thick upon standing.