Crispy Potato Roast

Make sure an oven rack is dead center and place a 9″ cast-iron skillet on it. Preheat the oven to 375°F.
While the oven heats up, whisk together the butter, oil, garlic powder, kosher salt, and crushed red pepper flakes in a small bowl. Set it aside. 
With a sharp knife or mandoline, slice your potatoes as thinly as you can – not the long way, we want circles, not strips. If you’re using a mandoline put it at 1/16th of an inch. After you slice each potato completely, carefully pick it up off the counter and use your hands to keep the slices held together so the potato keeps its shape: this will make it easier to arrange them in the skillet. 
When the oven is preheated carefully pull the skillet out of the oven, then brush the bottom and sides with a generous amount of the butter mixture, then arrange the thyme sprigs along the bottom of the skillet, spreading them out.
Arrange the potatoes in the pan, all along the outer edge. After the edges are filled, use your fingers to spread the slices out a little bit, just to fill in any extra space. Make sure they are tightly together. Next, fill the center with potatoes, then spread the slices out until there is no empty space remaining. Baste the top of the potato slices generously with the remaining butter mixture.
Bake for 90 minutes, or until the potato slices are tender and the tops are crispy and deeply browned.