Preheat your oven to 400 degrees F.
Spray a 9x13 inch pan with nonstick spray, or grease with butter.
Dump the bag of frozen hashbrowns into the pan. (There is no need to thaw first.)
Melt a stick of butter in a small bowl, and pour evenly over the potatoes. Sprinkle the potatoes with salt and pepper. Use a spoon to gently toss it all together, then spread it out so that it's even.
Bake at 400 degrees for 25-30 minutes, until the potatoes are tender and lightly browned on top. You can even nab a forkful to make sure they are done if you like.
Remove the casserole and reduce the oven temperature to 350.
Layer 1 and 1/2 cups each Monterey Jack and Cheddar cheese over the top of the potatoes (make sure you pack in your cheese! Don't skimp now!) Add the ham. You can either leave it layered or use a spoon to gently toss the cheese and potatoes and ham together. Either way is fine. (I like to toss for a more even distribution.)
In a large bowl or stand mixer, whisk together 8 eggs, 1 and 1/3 cups evaporated milk OR cream, 1 teaspoon seasoned salt, 1/2 teaspoon kosher salt (use less if all you have is table salt!), 1/2 teaspoon pepper, 1/4 teaspoon dry mustard, and 1/4 teaspoon onion powder. Beat well.
Pour the egg mixture evenly over the top of the casserole, making sure everything gets wet.
Bake at 350 degrees for about 40 minutes. It is done when bubbly. The edges should be starting to brown and the center should not wiggle when you shake the pan.
If the center of the casserole looks very wobbly but the top is starting to brown too much, cover with foil for the last few minutes of baking.