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

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

In a skillet over medium heat, sauté diced yellow onion in melted butter until softened, then add minced garlic, flour, spices, chicken broth, and milk, whisking until thickened.

Pour the sauce into the potato mixture and stir until well combined, then transfer to the prepared baking dish.

Bake for 45-50 minutes until bubbling and edges are golden brown.

In the meantime, heat vegetable oil in a frying pan and fry sliced onions until golden brown and crispy, about 10-15 minutes.

Sprinkle the crispy fried onions over the baked casserole once out of the oven.

Let cool for 5-10 minutes before serving.