Heat oven to 350. Coat a 13x9" glass baking dish with cooking spray.
Place potatoes in a large pot with enough water to cover. Lightly salt and bring to a boil. Lower to simmering; cook for 15 - 20 minutes or until fork tender, drain.
In a large bowl, mash potatoes with a hand mixer. Add butter, half & half, cream cheese, garlic salt and nutmeg. With mixer, beat potatoes on med-high speed until very smooth. Spoon into prepared baking dish.
Bake at 350, uncovered, for 30 minutes. Sprinkle sliced almonds over casserole; bake an additional 15 minutes, uncovered, until lightly browned. Let stand 15 minutes before serving.