Preheat the oven to 350°F and spray a 9×13 inch baking dish with nonstick spray.

Wash, peel, and slice the potatoes very thin using a mandolin for uniform slices. Set aside.

In a large saucepan over medium-low heat, melt the butter. Add the diced onion and minced garlic, and sauté until softened and fragrant.

Stir in the flour and cook for 2 more minutes, stirring constantly to form a roux.

Reduce the heat to low and slowly whisk in the chicken broth, followed by the milk. Increase the heat to medium and continue stirring until the sauce thickens and begins to boil.

Season the sauce with salt, pepper, and the dried thyme. Remove from heat and stir in the shredded cheddar cheese until melted and smooth.

Layer half of the sliced potatoes evenly in the prepared baking dish. Season with salt and pepper. Pour half of the cheese sauce over the potatoes.

Add the second layer of sliced potatoes. Season again with salt and pepper, and pour the remaining sauce on top. Sprinkle with the fresh thyme.

Cover the dish with foil and bake for 45 minutes. Then remove the foil and bake for an additional 30 minutes, until the top is golden and bubbly.

Let the scalloped potatoes rest for 10 to 15 minutes after baking to allow the sauce to thicken. Garnish with more fresh thyme before serving.