Preheat oven to 375 degrees
Spray 9x13 casserole dish with cooking spray- set aside
Use a mandolin & CAREFULLY slice peeled potatoes into rounds about 1/4" thick
Place sliced potatoes in a bowl of water
In a large saucepan, saute butter with onions until onions are soft, then add in garlic & cook an additional 1-2 minutes
Add in milk & half & half, heat to a simmer- remove from heat & set aside
Lay out paper towels & transfer potato slices out of water to towels & pat dry
Place a layer of potatoes in the bottom of the prepared baking dish to just cover the bottom
Sprinkle with salt & pepper & 1 cup cheese & bacon crumbles
Add another layer of dried potato rounds & repeat with salt, pepper, cheese & bacon
Continue until you have used all the ingredients - finishing with cheese layer
Pour hot milk mixture over the top & push potatoes down to submerge (they won't be fully covered) Sprinkle with remaining bacon
Cover with foil tightly & bake 1 hour
Remove foil, return to oven & bake another 40-45 minutes or until golden & bubbly
Potatoes should be soft when pierced with a fork