1. Heat oven to 375 degrees . Lightly coat a 3-quart baking dish with nonstick cooking spray.
2. Combine half-and-half, nutmeg and cayenne in a large pot; add potatoes and bring to a boil. Lower heat and simmer for 5 minutes. Drain potatoes over a large bowl, reserving half-and-half.
3. Meanwhile, melt butter in a large skillet over medium heat; add leeks and scallions. Cook for 5 minutes, stirring occasionally.
4. Layer one-third of the potatoes in prepared baking dish, overlapping as necessary. Sprinkle with 1/4 tsp of the salt, 1 tbsp of the flour, half the leek mixture and 1 cup of the cheese. Repeat layering. Top with remaining third of the potatoes and 1 cup cheese.
5. Pour half-and-half over potatoes; tilt dish from side to side to evenly distribute. Cover and bake at 375 degrees for 30 minutes; uncover and bake for an additional 30 minutes or until potatoes are tender and top of casserole is lightly browned.
6. Cool slightly before serving.