Preparation

1. Preheat the oven to 350 degrees.
2. Butter a shallow one-quart casserole or gratin dish.
3. Peel the yams and cut them in slices about 1/3-inch thick.
4. Arrange the slices, slightly overlapping, in a single layer in the dish. Sprinkle with sage, nutmeg, salt, and pepper.
5. Dot with butter.
6. Bake in the preheated oven for about 1 1/2 hours, until the yams are slightly tender.
7. Top the gratin with cheese and bake for 30 minutes more, until the yams are very tender and the cheese is melted and golden.