Cook bacon until lightly crispy.  Add butter and onion and cook until the onion is soft.

Add salt, pepper, and thyme; then the flour or arrowroot.

Add milk to the bacon mixture, cook, and stir until thickening.

Chop your potatoes and put them in a dish (I'm doing a half batch here).

Add the sour cream.

Add the cheeses, reserving a small amount of cheddar for the top.   Pour the white sauce over it, too.

Stir it all up and add a little cheddar to the top.

Bake at 350 for an hour and a half or until the edges are bubbly and browned.  A half batch like this one only needs 45 - 60 minutes.