Heat oven to 375°F; grease a shallow 2-quart dish with 1 teaspoon butter. Drain washed leeks well.
Melt remaining butter in a wide, heavy saucepan over medium. Cook leek 7 minutes until soft, not browned. Add cream, milk, salt, pepper, and nutmeg; barely simmer.
Add freshly sliced potatoes without rinsing starch off. Gently simmer 8 to 10 minutes, carefully lifting and turning, until sauce thickens slightly and potato edges soften.
Spread potatoes and cream in even layers in dish. Cover with foil; bake 20 minutes.
Uncover; bake 15 to 20 minutes until golden and knife slides through center easily. Rest 10 minutes.