1 1/2 pounds (about 2 large) russet potatoes, peeled, cubed
1 savoy or napa cabbage, trimmed, pale-green leaves finely shredded (4 cups)
1 leek, pale-green and white parts only, cut into 1/2-inch dice
1 cup milk
6 tablespoons butter, melted
1/4 teaspoon nutmeg
Preheat broiler. Place potatoes in a medium saucepan; add enough cold water to cover and some sea salt. Bring to a boil over high heat; reduce heat to a simmer, and cook until tender, about 15 minutes. Drain potatoes and return to saucepan. Mash with a potato masher; cover pan to keep warm.
Meanwhile, in another saucepan, combine cabbage, leek, milk, 2 tablespoons butter, and nutmeg; season with salt. Cover, and cook over medium heat, stirring occasionally, until cabbage and leek is soft but not browned, about 15 minutes. Stir into potatoes. Taste and add additional salt if desired.
Spread mixture in an 8-inch square baking dish. Make a small well in the center, and place under the broiler until lightly browned on top, about 5 minutes. Remove from broiler. Place remaining 4 tablespoons butter in well. Serve immediately, spooning melted butter from well onto each serving, if desired.