Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking dish thoroughly to prevent sticking during baking. I find using a bit of butter or a non-stick spray works best.

Place the frozen hash brown patties in the prepared baking dish, arranging them in a single even layer across the bottom.

Evenly sprinkle the shredded sharp cheddar cheese, Monterey Jack cheese, diced cooked ham, and chopped scallions over the hash brown patties. Distribute each ingredient so every bite is flavorful.

In a large bowl, whisk together the eggs, whole milk, salt, ground dry mustard (if using), and garlic powder until well combined. The mixture should be smooth and fully blended. I like to whisk for an extra minute to ensure the eggs and milk are well incorporated.

Pour the egg custard evenly over the ham and cheese mixture in the baking dish, making sure the liquid covers everything. Cover the dish with foil and bake in the preheated oven for 25 minutes. After 25 minutes, carefully remove the foil and continue baking for another 20 minutes, or until the edges are golden and a knife inserted in the center comes out clean.

Remove the casserole from the oven and allow it to cool for 5-10 minutes before slicing into portions and serving. Allowing it to rest makes it easier to cut and helps the flavors set. I like to top mine with a few extra chopped scallions for brightness.