Coat a 3 quart baking dish with cooking spray. Spread frozen potatoes evenly in the bottom of the prepared baking dish. Sprinkle with ham, cheese and onion
In medium bowl, mix eggs, milk, mustard, salt and black pepper. Pour egg mixture evenly over layers in baking dish. Cover and chill for at least 2 to 24 hours.
Preheat oven to3 50. Bake uncovered, for 55 to 60 minutes or til a knife inserted near center comes out clean. Sprinkle with green onions. Let stand for minutes before serving. Serve with horseradish sour cream. Serves 8
Horseradish Sour cream: in small bowl, stir together 1/2 cup sour cream, 1 to 2 Tbsp. horseradish mustard and 1 Tbsp. snipped fresh chives.