Add bacon, 3 cups chicken stock, potatoes and onion to the bowl of a large slow cooker, and stir to combine. Cook on low for 6-8 hours or on high for 3-4 hours, or until the potatoes are completely tender and cooked through.
Once the soup has slow cooked and is about ready to serve, cook the butter in a small saucepan on the stove over medium-high heat until it has melted. Whisk in the flour until it is completely combined, and then cook for 1 minute, stirring occasionally. Gradually add in the evaporated milk while whisking it together with the flour mixture, and continue whisking until the mixture is completely smooth. Let the mixture continue cooking until it reaches a simmer, stirring occasionally, and then it should get really thick.
Immediately pour the milk mixture into the slow cooker with the potatoes, and stir until combined. Add in the cheddar cheese, Greek yogurt (or sour cream), salt and pepper, and stir until combined. If you would like the soup to be even thicker, you can use a potato masher or a large spoon to mash about half of the potatoes (while the soup is still in the slow cooker) to thicken the soup up. If you would like the soup to be thinner, add in an extra 1-2 cups of warmed chicken or vegetable stock. Stir to combine, then taste and add more salt and pepper if needed.
Serve warm, garnished with desired toppings. Or transfer to a sealed container and refrigerate for up to 3 days. (This recipe will not freeze well.)
