Preheat the oven to 350 degrees F and grease the bottom of a 9×13 baking dish.
Add frozen shredded hash browns in a single layer. Season with a little salt and pepper to add additional flavor, if preferred. Top with cheeses, ham, and diced scallions.
In a large separate bowl, whisk together eggs, milk, salt, pepper, and garlic powder, seasoning to taste, then pour mixture evenly over the casserole.
Cover with foil and bake in the oven for 25 minutes, then uncover and bake for another 15-20 minutes, or until the cheese is melted and everything is cooked through.
Allow to cool for 5-10 minutes, then slice and serve!