Sweet Potato Pommes Anna (Layered Potato Casserole)

Preheat oven to 400 degrees F.

Combine butter and oil in a small dish. Liberally brush the bottom and sides of an 8-inch cast-iron skillet with some of the mixture.

Using a mandoline, thinly slice sweet potatoes about 1/16 inch thick. Arrange one-fourth of the potatoes in concentric circles in the pan, overlapping the slices and alternating colors. Brush with some of the butter mixture and sprinkle with 1/4 teaspoon each thyme, salt and pepper. Repeat to make three more layers.

Heat the pan over high heat until the sweet potatoes start sizzling, about 3 minutes. Cover the pan with foil and transfer to the oven. Bake for 30 minutes.

Uncover and continue baking until the sweet potatoes in the center are easily pierced with a skewer and the bottom is golden brown and slightly crispy, 20 to 25 minutes more.

Let cool for 10 minutes. Run a small sharp knife around the edge of the pan to loosen the potatoes, then invert the galette onto a serving plate. Cut into 8 wedges.