1. Preheat your oven to 350°F. Prepare the potatoes by peeling them and either using a mandoline slicer or cutting them into thin slices.
2. Add the butter to a saucepan over medium-high heat, add the garlic and cook it for about 30 seconds. Sprinkle in the flour and whisk until incorporated. Slowly add in the cream, milk, salt, and some pepper. Let it thicken on low heat.
3. Add in the cheeses and let melt, stirring occasionally. Remove from heat.
4. Arrange half of the potatoes on the bottom of a greased 9x13 baking dish. Pour half of the cheese mixture over top. Repeat the process for the second layer.
5. Bake, covered with foil, for 40 minutes, then remove the foil and bake for another 15 minutes or until a toothpick slides into the potatoes without much resistance. Let the potatoes sit for 5-10 minutes prior to serving.