Preheat the oven to 350°F.
Place leeks in a very large bowl of water. Separate them into rings and swish around in water to loosen the dirt. Strain off the water and rinse briefly, then allow excess water to drain off. (A salad spinner works great for this.)
In a 10-inch skillet, heat 2 Tablespoons of the olive oil over medium heat. Add leeks. Cook, stirring frequently, until leeks start to soften. Add a little water, if needed, to prevent them from burning. Season with a little salt and pepper. Add the minced fresh sage, and cook for another 1-2 minutes.
Lightly oil the bottom and sides of an 8- by 11-inch baking dish. Arrange 1/2 of the sweet potato slices over the bottom of the baking dish. Season well with salt and pepper. Top with half of the leeks and sprinkle with a little freshly grated nutmeg. Repeat with remaining sweet potato slices and leeks, again seasoning well with salt and pepper, and a sprinkle of fresh nutmeg.
Cover the top of the dish tightly with aluminum foil. Bake for 40-45 minutes. Remove from oven and uncover. Sprinkle cheese over the top. Raise the oven temperature to 450°F and bake for another 10 minutes, or until the cheese is golden brown. Let cool for a few minutes before serving.