2.5 lbs russet potatoes
12 tablespoons unsalted butter
2 teaspoons coarse kosher salt
Freshly ground black pepper, to taste
Preheat oven to 350°F.
Clarify the butter: Melt butter in the No. 10 Chef Skillet over medium-low heat. When butter is bubbling and the foaming subsides, remove from the heat. Skim any foam off the top with a spoon. Carefully pour warm butter through a fine mesh sieve into a heatproof bowl, leaving the white milk solids behind. You should have about 1/2 cup clear yellow (clarified) butter.
Peel the potatoes. Using a mandolin or a sharp knife, carefully slice potatoes crosswise into very thin (⅛-inch) rounds. Arrange half of the potato slices in the No. 10 Chef Skillet in concentric circles, starting at the center and working your way to the edge. (Don’t wipe out the skillet; you want that good buttery coat on the bottom.) Pour half of the clarified butter over the top then season with 1 teaspoon salt, 1 teaspoon thyme leaves, and freshly ground black pepper. Continue arranging remaining potato slices in concentric circles, then pour the rest of the clarified butter over the top. Season with 1 teaspoon salt, 1 teaspoon thyme, and freshly ground black pepper.
Cover with a lid or aluminum foil and transfer to the preheated oven for 30 minutes.
After 30 minutes, remove the lid or foil and increase oven temperature to 425˚F. Continue to cook until the top layer is golden brown and potatoes are creamy tender, an additional 30-40 minutes.