Spray a 9x13 pan with nonstick spray. Preheat oven to 350 degrees.
Wash, dry and peel the potatoes. Using a mandolin slice the potatoes very thin (1/8 inch).
In a large saucepan over medium low heat melt the butter. Add the onion and garlic and cook until the onions and garlic are soft.
Add the flour and cook 2 minutes longer.
Reduce the heat to low and slowly pour in the chicken broth whisking the whole time. Next add the milk and continue whisking. Increase the heat to medium and stir constantly until the mixture comes to a boil and thickens. Season with salt and pepper to taste.
Remove from heat and stir in the ½ teaspoon of thyme and the cheddar cheese until the cheese melts.
Place half of the sliced potatoes in a layer in the bottom of the 9x13 prepared pan. Season with salt and pepper. Pour half of the sauce over the top of the potatoes.
Next add the second layer of sliced potatoes. Season with salt and pepper. Pour the remaining sauce over the top. Sprinkle with fresh thyme.
Cover with foil and bake for 45 minutes.
Remove the foil and bake another 30 minutes.
Remove from oven. Garnish with fresh thyme. Let the dish sit for about 10-15 minutes before serving.