Prepare Potatoes- rinse potatoes with water and pat dry. Using a mandoline slicer, carefully slice potatoes to a thickness of 1/8 inch. Set slices aside.
Make Cheese Sauce- Grate and mix all cheeses in a mixing bowl. Melt butter in a large pot over medium-low heat. Add garlic and cook 1-2 minutes until fragrant. Sprinkle flour over garlic and butter and whisk until a light in color paste forms. Slowly whisk in chicken broth until smooth. Whisk in cream, ground nutmeg, kosher salt, and a few cracks of pepper. Continue whisking for a few minutes until mixture has thickened slightly. The consistency should be like a thin-medium gravy. (If flour & cream mixture doesn't start to thicken after 2 minutes, turn heat up slightly an continue whisking.) Once thickened, remove pot from heat and fold in 1 cup of grated cheese, a handful at a time. Taste and add more salt if needed.
Construct & Bake- Preheat oven to 375F. Meanwhile, grease an oven safe 9 inch round baking dish with butter. Cover bottom of baking dish with 1/3 of sliced potatoes in a single, slightly overlapping layer. Top potatoes with half of cheese sauce. Repeat with another 1/3 potatoes and remaining cheese sauce. Add last layer of potatoes and top with remaining 1/2 cup grated cheese. Cover with foil and bake 60-90 minutes until potatoes are tender. For an extra crispy cheese topping, pop dish under the broiler for a minute or two. Keep an eye on it so it doesn't burn. Sprinkle with a bit of fresh parsley and fresh cracked pepper.
Notes
If you don't have a broiler, uncover potatoes about 20 minutes before removing from the oven to brown the top cheese layer.
This recipe can be doubled and baked in a 9x13 casserole dish.
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