Preheat your oven to 350°F (175°C). Grease a 13x9-inch (3-quart) glass baking dish with cooking spray.
In a 10-inch skillet, cook the diced bacon and chopped onion over medium-high heat for approximately 10 minutes, or until the bacon is crispy and the onion is tender. Drain the mixture on paper towels.
In a large bowl, combine the bacon mixture, thawed hash brown potatoes, shredded Cheddar cheese, and grated Parmesan cheese. Mix well, ensuring all ingredients are evenly distributed. Spoon the mixture into the prepared baking dish.
In the same bowl, beat the eggs, milk, sour cream, salt, ground mustard, and pepper using a fork or wire whisk until well blended. Pour this egg mixture over the potato mixture in the baking dish.
Bake the casserole uncovered for 35 minutes.
In a small bowl, toss the cornflake crumbs (or panko bread crumbs) with the melted butter until evenly coated. Sprinkle this mixture over the casserole.
Return the casserole to the oven and bake for an additional 10 to 15 minutes, or until a knife inserted in the center comes out clean.
Allow the casserole to cool slightly before serving. Enjoy!