Spray slow-cooker well with non-stick spray or use liner
Wash, peel and slice potatoes, peel and slice onion
Shred cheeses and set aside.
Layer potatoes and onions in slow-cooker. -Salt and Pepper to taste (I use a lot )
In a sauce pan on medium high heat melt butter. Wisk in flour and garlic powder , stir constantly, slowly add milk ,as it begins to thicken, stir in sour cream.
Bring white sauce to boil , reduce heat , stir in cheeses and more salt and pepper and any other spices you may want.
When cheese is completely melted, pour sauce over the layered potatoes.
Note!!! I did not pre-cook onions, however if you prefer them very soft you can saute in olive oil before layering between potatoes.
Make sure sauce is distributed well over all potatoes.
Set slow-cooker on high and cook for 3-4 hours or until tender.
TIP !! Layer 3-4 paper towels and lay across slow-cooker before placing lid on. This will absorb excess moisture. Change towels half way through.
Once you hit 3 hours watch closely until desired doneness.
Paper towel tip credited to Chelsea at www.chelseasmessyapron.com/