Preheat your oven to 350°F (175°C) and coat a 9x13-inch baking dish with nonstick spray.

In a large bowl, combine thawed cubed hash browns, melted unsalted butter, sharp cheddar cheese, and sour cream until well mixed.

In a medium skillet over medium heat, melt a tablespoon of butter, then add diced yellow onions and sauté for 3-5 minutes until softened.

Stir in minced garlic and cook for another minute.

Sprinkle flour, salt, black pepper, cayenne pepper, and dried mustard into the onions, and cook for another minute.

Gradually whisk in chicken broth and whole milk, cooking for 3-5 minutes until sauce thickens.

Pour the sauce over the potato mixture, fold gently, and transfer to the prepared baking dish. Bake for 45-50 minutes.

While baking, heat vegetable oil to 350°F (175°C) in a large saucepan, and fry sliced onions until golden brown and crispy.

Remove the casserole from the oven, top with crispy fried onions, and serve warm.