01 -
Preheat oven to 175°C. Grease a 23×33 cm baking dish with nonstick spray.
02 -
In a large bowl, thoroughly combine thawed hash browns, melted butter, shredded cheddar cheese, and sour cream. Set mixture aside.
03 -
Melt butter in a medium non-stick skillet over medium heat. Add diced onions and sauté until softened, 3–5 minutes. Stir in minced garlic and cook for 1 minute. Add flour, salt, black pepper, cayenne pepper, and dried mustard, stirring constantly for 1 minute.
04 -
Gradually pour in whole milk and chicken broth, whisking continuously until fully combined. Continue whisking and cook until the sauce thickens.
05 -
Pour the finished cheese sauce over the potato mixture. Stir to evenly combine, then transfer to the prepared baking dish.
06 -
Bake in the preheated oven for 45–50 minutes, or until bubbling and thoroughly heated.
07 -
While the casserole bakes, line a large baking sheet with two layers of paper towels. Heat vegetable oil in a large saucepan over high heat until it reaches 175°C. Working in batches, fry about 1 cup of sliced onions at a time, stirring occasionally, until pale golden brown, 2–5 minutes. Remove onions using a slotted spoon and place on the lined baking sheet. Repeat with remaining onions, ensuring the oil returns to temperature between batches.
08 -
Immediately sprinkle the fried onions with kosher salt and toss to coat evenly.
09 -
Once the casserole has finished baking, remove from the oven and evenly distribute the crispy fried onions over the top. Serve hot.