Cheesy Potato Casserole

INGREDIENTS:
2 pounds 32 ounces frozen shredded hash brown potatoes
2 cups 8 ounces shredded sharp cheddar cheese
1 cup sour cream
1 can 10.5 ounces condensed cream of chicken soup
1/2 cup 1 stick / 8 tablespoons unsalted butter, melted
1/2 cup whole milk
1/2 medium yellow onion very finely diced (about 1/2 cup)
1 teaspoon kosher salt
1/2 teaspoon black pepper
1/2 teaspoon garlic powder
1/2 teaspoon onion powder
2 cups crushed cornflakes
2 tablespoons unsalted butter melted
1/4 teaspoon kosher salt
1/2 cup cooked crumbled bacon
1/2 cup diced ham
1/4 cup sliced green onions
1/2 cup shredded mozzarella or Monterey Jack
1 –2 tablespoons chopped fresh parsley or chives
3 tablespoons unsalted butter
3 tablespoons all-purpose flour
1 1/2 cups chicken broth or vegetable broth
1/2 cup whole milk or half-and-half
1/2 teaspoon kosher salt
1/4 teaspoon black pepper
1/2 teaspoon garlic powder