Preheat your oven to 350°F (175°C) and coat a 9×13-inch baking dish with cooking spray

Remove the frozen hash browns from the freezer and allow them to thaw completely

Pat the thawed potatoes with paper towels to remove excess moisture

In a large mixing bowl, combine the thawed hash browns, melted butter, shredded cheddar cheese, and sour cream

Stir until all ingredients are well incorporated and set aside

In a medium non-stick skillet over medium heat, melt the butter until it begins to foam

Add the diced onions and sauté until they become translucent and soft, about 3-5 minutes

Add the minced garlic and cook for an additional minute, stirring constantly to prevent burning

Sprinkle the flour, salt, black pepper, cayenne pepper, and dried mustard over the onion mixture

Cook for another minute, stirring continuously to create a roux

Gradually whisk in the chicken broth and milk, ensuring no lumps form

Continue cooking and whisking until the sauce thickens to a gravy-like consistency, about 2-3 minutes

Pour the hot sauce over the potato mixture and fold gently until everything is evenly coated

Transfer the combined mixture to the prepared baking dish, spreading it out evenly

Bake uncovered for 45-50 minutes, until the edges are bubbly and the center is heated through

While the casserole bakes, prepare the crispy onions

Line a large baking sheet with a double layer of paper towels

Heat the vegetable oil in a large, heavy-bottomed pot to 350°F (use a candy or deep-fry thermometer)

Working in batches of about 1 cup at a time, carefully add the sliced onions to the hot oil

Fry for 2-5 minutes, stirring occasionally with a slotted spoon, until the onions turn golden brown

Using a mesh strainer or slotted spoon, transfer the fried onions to the paper towel-lined baking sheet

Immediately sprinkle with salt while still hot

Repeat with remaining onions, ensuring the oil returns to 350°F between batches

Once the casserole has finished baking, remove it from the oven

Immediately top with the crispy fried onions, distributing them evenly across the surface

Serve hot while the onions are still crispy