Crisp the Bacon: In a heavy pot, cook bacon over medium heat until crispy. Remove and drain on paper towels. Leave 2 tablespoons of bacon fat in the pot.
Build the Base: Add butter and chopped onion to the pot. Sauté until soft and translucent, about 5–7 minutes. Add garlic and cook for 30 seconds.
Make the Roux: Sprinkle flour over onion mixture. Stir and cook 1–2 minutes until golden. This step prevents the floury taste later.
Add Liquids & Potatoes: Gradually whisk in chicken broth and milk. Stir in both types of potatoes — mashed baked russets and diced golds. Simmer gently for 15–20 minutes.
Stir in Sour Cream & Cheese: Reduce heat and stir in sour cream and shredded cheddar. Stir until melted and creamy. Season to taste with salt and pepper.
Serve Loaded: Ladle into bowls and top with reserved bacon, extra cheese, scallions, and a dollop of sour cream.