Preheat oven to 350 degrees F (175 degrees C). Spray a 9x13-inch baking dish with non-stick cooking spray.
Heat a large Dutch oven over medium-high heat and melt Unsalted Butter (6 tablespoon). Add Onion (1) and saute, stirring occasionally, until the onions start to soften. Sprinkle All-Purpose Flour (1/4 cup) over the onions and stir until well blended. Cook for 2 minutes, stirring constantly.
Whisk in the Milk (1 cup) and continue whisking until the mixture is blended and lump-free. Add the Chicken Broth (2 cup) and whisk again. Bring mixture to a simmer and allow to thicken, which will take about 3 minutes. Season to taste with Salt (to taste) and Ground Black Pepper (to taste).
Remove from the stove and stir in the Shredded Sharp Cheddar Cheese (2 cup), Monterey Jack Cheese (1 cup), and Sour Cream (1 cup), stir until melted and blended. Add the Frozen Shredded Hash Browns (1 bag) and stir until evenly mixed.
Spread potato mixture evenly in prepared baking dish. Add Cooked Bacon (to taste) if desired.
In a small bowl, combine the Unsalted Butter (6 tablespoon), Corn Flakes (3 cup), Grated Parmesan Cheese (1/4 cup), and Smoked Paprika (2 teaspoon). Stir gently until the topping is evenly coated and crumbly. Sprinkle evenly over top of potato mixture.
Cover baking dish with foil and bake for 25-30 minutes (it will take the longer time if your hash browns were completely frozen).
Remove foil and bake for an additional 15 minutes or until golden and bubbling around the edges and topping is crispy.
Remove from oven and let rest for 10 minutes before serving; garnish with Fresh Parsley (to taste).