Pierce the potatoes multiple times with a fork, then microwave them for 12 to 15 minutes, or until tender.* Carefully halve the potatoes and let cool. Once cool enough to handle, remove the skins, and cut into chunks.
In a large pot, melt the butter over medium-low heat. Add the reserved bacon fat, garlic and onion and cook for 2 to 3 minutes, or until the onion is tender. Slowly whisk the flour into the pan and stir for 1 to 2 minutes. Slowly whisk in the milk and half-and-half. Keep whisking until smooth.
Gradually add the chicken stock. Bring to a light simmer and whisk in the kosher salt, garlic salt and pepper. Keep at a light simmer until the mixture has thickened slightly, 5 to 7 minutes.
Stir in the remaining cheeses, remaining bacon, and the sour cream. Remove the pot from the heat. Scoop potato chunks into the pan, breaking them into small pieces or leaving them chunky, depending on your preference.
Serve hot, topped with your favorite garnishes, like cheese, bacon and chives.