Preheat the oven to 375℉ and lightly grease a 9x13-inch baking dish with butter or nonstick cooking spray.

Wash and peel potatoes. Cut them into thin, slices about ⅛ inch thick using a mandoline slicer. Don't rinse the potatoes after slicing because the starch helps thicken the sauce.

Melt the butter in a medium saucepan over medium heat. Add the minced garlic and cook for about 30 seconds, just until fragrant. Whisk in the flour and cook for another minutes, stirring constantly.

Slowly pour in the whole milk whisking continuously and then slowly pour the heavy cream whisky continuously. Cook for about 3-5 minutes, stirring frequently, until it starts to thicken.

Remove the saucepan from the heat and stir in 2 cups of shredded cheese (save the remaining for the topping) and stir until melted. Add the salt, pepper, thyme and onion powder and stir.

Arrange half of the sliced potatoes in the prepared baking dish, overlapping them slightly. Pour half of the cheese sauce over the potatoes. Then, add the remaining potatoes and pour the rest of the sauce evenly over top.

Sprinkle the remaining shredded cheese over top, cover with foil and bake for about 45 minutes, until bubbly.

Remove foil and bake for another 20 minutes or until top is a light golden brown and the potatoes are fork tender.

Remove from oven and let it rest for about 10-15 minutes before serving.