Preheat oven to 350 degrees F. Grease a 9×13 baking dish.
Arrange frozen hash brown patties in the dish, in a single layer, and sprinkle the cheeses, ham, and scallions evenly over the top.
In a large bowl, whisk together the eggs, milk, salt, mustard, and garlic powder; pour evenly over the ham mixture.
Cover with foil and bake for 25 minutes. Remove foil and bake for another 20 minutes or until the edges are golden and a knife inserted in the center comes out clean.
Allow to cool for 5-10 minutes, then cut into portions and serve
