Prick the potatoes all over with a fork and microwave for 12 to 15 minutes, until tender; halve them and let them cool before peeling and cutting into chunks. A fork should pass easily through the center, and the potatoes should be cool enough to handle without rushing.

Cook the bacon in a skillet over medium-high heat until crisp, then move it to a paper towel-lined plate and save up to 1 tablespoon of drippings before crumbling the cooled bacon. The bacon should look deeply browned and feel crisp as it cools, with most of the excess fat left behind.

Melt the butter in a large pot over medium-low heat, then cook the reserved drippings, garlic, and onion for 2 to 3 minutes; whisk in the flour for 1 to 2 minutes before gradually whisking in the milk, half-and-half, stock, salt, garlic salt, and pepper, then lightly simmer for 5 to 7 minutes. The flour mixture should turn pale golden and smell lightly toasted before the liquids go in, and the broth should slowly thicken into a smooth coating on the whisk.

Reserve 1/4 cup each of the cheese and bacon if using them as garnishes, turn off the heat, then stir in the remaining cheese, bacon, and sour cream before breaking the potato chunks into the pot. The broth should look glossy and unified after the dairy melts in, with potato pieces left as chunky or as broken down as you prefer.