Slice the potatoes thinly and evenly, using a mandoline or sharp knife, and place them in a large bowl. Season with olive oil, kosher salt, garlic powder, and black pepper, tossing until every slice is lightly coated.

Grease a rimmed sheet pan or shallow baking dish and arrange the potatoes in tight, overlapping rows, alternating varieties and filling the pan completely.

Drizzle the melted butter over the potatoes and scatter the thyme across the surface.

Cover the pan with foil and bake until the potatoes begin to soften, about 30 minutes.

Uncover and return to the oven, baking until the top is well browned and crisp, 35 to 40 minutes more. Increase the oven temperature to 425°F/220°C during the final 10 minutes for deeper color.