Preheat oven to 350° F. Lightly grease a 9x9-inch baking dish, or 6 cup gratin dish with vegetable cooking spray or butter. Place the baking dish on a cookie sheet. Set aside.
In a medium saucepan melt 2 tablespoons butter over medium heat. Add the flour and cook, stirring constantly for 3 to 4 minutes. Add the salt, pepper, nutmeg, cayenne, garlic powder and half-and-half. Cook, stirring constantly, about 3 to 5 minutes or until it starts to thicken. Remove from the heat, cover to keep warm.
Peel and slice the potatoes to 1/16 to 1/8-inch thickness. Layer ⅓ of the potatoes in the prepared baking dish. Spoon ⅓ of the sauce over the potatoes. Sprinkle with ¼ cup of the cheese.
Repeat layering ½ of the remaining potatoes, ½ the remaining sauce and sprinkle with the remaining ¼ cup Gruyère.
Top with the remaining potatoes then pour all the remaining sauce over the top.
Bake, uncovered, for 60 minutes. * If the top is drying out too quickly, remove the casserole from the oven and gently press down with a large flat spatula submerging the potatoes in the sauce. Tent with foil, if needed, and continue baking.
After 1 hour of baking, sprinkle the potatoes with the remaining 3 tablespoons of cheese. Return the pan to the oven and cook another 30 minutes or until the potatoes are fork tender and the top is well browned.
Remove from the oven and allow the potatoes to rest 15 minutes before serving. Garnish with parsley, if desired.