Heat the oven to 375°F and grease a shallow 2-quart baking dish with 1 teaspoon of the butter. Drain washed leek slices very well.
Melt remaining butter in a wide heavy saucepan over medium heat and cook leek for 7 minutes until soft without browning. Add cream, milk, salt, pepper and nutmeg and bring just to a simmer.
Add the freshly sliced potatoes without rinsing away their starch. Simmer gently for 8 to 10 minutes, lifting and turning the slices carefully, until the sauce lightly thickens and the outer edges of the potatoes soften.
Transfer potatoes and cream to the baking dish, spreading the slices into even layers. Cover with foil and bake for 20 minutes.
Uncover and bake for 15 to 20 minutes until golden and a thin knife slides easily through the center. Rest for 10 minutes before serving.