Gather all ingredients and equipment needed.

Preheat oven to 350°F (176°C).

Using a mandolin or a large sharp knife, slice red potatoes thin. Then evenly layer sliced potatoes in a casserole dish.

To make the roux, in a small pot or skillet, on low medium heat, add in butter and let melt, then add in flour and whisk together until mixed. Then, set aside.

In a large pot, on medium heat, add milk and slowly let it come to a boil and then reduce heat to a simmer. Then add in white roux while continuously whisking. As soon as sauce begins to thicken add in grated parmesan cheese, granulated garlic, salt, and white pepper. Let sauce continue to simmer until sauce is thick.

Then pour the sauce over on top of evenly layered slice red potatoes until covered with sauce. Then place casserole dish into preheated oven at 350°F (176°C) for 45 minutes or until potatoes are fork tender and top is light golden brown in color.

Remove from oven and sprinkle fresh thyme leaves and let cool before serving.