Heat oven to 375ºF.

Melt 3 tablespoons of the unsalted butter in a large 12-inch high-sided or cast-iron skillet over medium heat. Add in the thinly sliced white part of 1 medium leek, 4 cups shredded green cabbage, and 1 teaspoon of the kosher salt. Cook, stirring occasionally, until the cabbage begins to brown, 6 to 8 minutes.

Stir in 4 minced garlic cloves and cook until fragrant, about 15 seconds. Transfer the cabbage mixture to a medium bowl. Reserve the skillet, no need to clean.

Bring 2 cups heavy cream, the remaining 1 teaspoon kosher salt, ¼ teaspoon black pepper, and ¼ teaspoon ground nutmeg to a simmer in a small saucepan over medium heat. Whisk in 1 ½ cups of the shredded Irish white cheddar cheese until the sauce is smooth. Bring to a boil. Remove the saucepan from the heat.

Using a sharp knife or a mandolin, thinly slice 2 pounds medium or large gold potatoes (about ⅛-inch thick, no need to peel). Coat the reserved skillet with the remaining 1 tablespoon unsalted butter (it’s okay if it melts).

Arrange half of the potato slices in an even layer in the skillet. Sprinkle half of the cabbage mixture over the potatoes. Layer the remaining potatoes over the cabbage. Pour the sauce evenly over the top. Sprinkle evenly with the remaining cabbage mixture and remaining ½ cup shredded cheddar cheese. Cover the skillet loosely with aluminum foil.

Bake for 40 minutes. Uncover and continue to bake until the top is browned and the potatoes in the center are knife tender, 20 to 30 minutes more. Let cool for 15 minutes before serving. Garnish with finely chopped fresh chives and black pepper if desired.