1 1/2 pounds russet potatoes
1 savoy 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
4 tablespoons unsalted butter
1/4 teaspoon freshly grated nutmeg
Preheat broiler. Peel and quarter potatoes, and place in a medium saucepan; add enough cold water to cover. Bring to a boil over high heat; reduce heat to a simmer, and cook until tender when pierced with a fork, about 15 minutes. Drain potatoes and return to saucepan. Mash with a potato masher or pass through a ricer; 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.
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 2 tablespoons butter in well. Serve immediately, spooning melted butter from well onto each serving, if desired.