1️⃣ Preheat the Oven:

Preheat the oven to 350°F (175°C).

Spray a 9×13-inch baking dish with nonstick spray.

2️⃣ Prepare the Potato Mixture:

In a large bowl, combine the hash browns, melted butter, shredded cheese, and sour cream. Mix well and set aside.

3️⃣ Make the Sauce:

In a medium, non-stick skillet over medium heat, melt the butter.

Add the diced onion and cook until softened, about 3-5 minutes. Add the garlic and cook for 1 more minute.

Stir in the flour, kosher salt, black pepper, cayenne pepper, and dried mustard. Cook for another minute.

Gradually whisk in the chicken broth and milk, whisking continuously until smooth.

Continue cooking, whisking often, until the sauce thickens.

4️⃣ Combine and Bake:

Pour the prepared sauce into the potato mixture and mix until well combined.

Spread the mixture evenly into the prepared baking dish.

Bake for 45-50 minutes, or until the casserole is bubbly and heated through.

5️⃣ Prepare the Fried Onions:

Line a large baking sheet with two layers of paper towels and set aside.

Heat the vegetable oil in a large saucepan over high heat until it reaches 350°F (175°C).

Working in batches, fry 1 cup of sliced onions at a time in the hot oil. Fry, stirring occasionally, until the onions are pale golden brown, about 2-5 minutes per batch.

Using a slotted spoon or fine mesh strainer, transfer the fried onions to the prepared baking sheet.

Sprinkle the fried onions with kosher salt while they are still warm, tossing to coat evenly.

6️⃣ Top and Serve:

Once the casserole has baked for 45-50 minutes, remove it from the oven.

Sprinkle the crispy fried onions evenly over the top of the hot casserole.

Serve immediately and enjoy!