Cook the Bacon in a large heavy pot or Dutch oven over medium heat until crisp; transfer to a paper towel-lined plate, reserving about 1–2 tablespoons of bacon fat in the pot and chopping the cooked bacon for garnish.
Add the Unsalted Butter to the pot with the bacon fat and melt, then add the diced Yellow Onion and sauté until softened and translucent, about 5 minutes. Stir in the minced Garlic and cook 30 seconds until fragrant.
Sprinkle the All-Purpose Flour over the onion mixture and cook, stirring constantly, about 1–2 minutes to remove the raw flour taste and form a light roux.
Slowly whisk in the Low-Sodium Chicken Broth, scraping up any browned bits from the bottom of the pot, until the mixture is smooth.
Add the peeled and diced Russet Potatoes to the pot, bring to a simmer, cover, and cook until the potatoes are very tender, about 15–20 minutes.
Use a potato masher or the back of a spoon to mash a portion of the potatoes in the pot to thicken the soup while leaving plenty of chunks for texture.
Stir in the Whole Milk and Heavy Cream and warm gently over low heat until heated through; do not boil.
Remove the pot from the heat and stir in the Sour Cream and Shredded Sharp Cheddar Cheese until the cheese is melted and the soup is smooth and creamy.
Taste and adjust seasoning with Kosher Salt and Freshly Ground Black Pepper.
Ladle the soup into bowls and garnish with the reserved chopped bacon, extra shredded cheddar, chopped Green Onions, and a dollop of sour cream if desired. Serve hot.
