Step 1

Wash potatoes and cut them in 1/8" thick slices, set aside.  *Use a mandoline slicer if you have one.

Step 2

Spray slow cooker with non-stick cooking spray, set aside.

Step 3

In a medium saucepan over medium-low heat melt the butter. Then whisk in the flour until the dry flour is no longer visible.

Step 4

Add the milk, garlic powder, salt and pepper, and whisk to combine. Continue to cook just until it starts to thicken, whisking frequently. Remove from heat and stir in cheeses until melted.

Step 5

Add 1/3 of the sliced potatoes in the bottom of the slow cooker.  Then add 1/3 of the chopped onion on top of the potatoes. Pour 1/3 of the cheese sauce over the potatoes and onions.  Repeat these layers 2 more times.

Step 6

Cover and cook on high for 3-4 hours or low for 4-6 hours or until potatoes are tender.

Step 7

Turn off the heat and let the potatoes sit for 20 minutes before serving for the sauce to thicken.