01 -
Preheat oven to 350°F. Coat a 9×13-inch baking dish with nonstick spray.
02 -
In a large bowl, mix thawed hash brown potatoes, melted butter, shredded sharp cheddar cheese, and sour cream until fully combined. Set aside.
03 -
In a medium nonstick skillet over medium heat, melt butter. Add diced onion and sauté until softened, about 3–5 minutes. Stir in minced garlic and cook for 1 additional minute. Sprinkle in flour, salt, black pepper, cayenne pepper, and dried mustard. Cook, stirring constantly, for 1 minute.
04 -
Slowly pour in whole milk and chicken broth while whisking continuously. Continue whisking over medium heat until the mixture thickens and is smooth.
05 -
Pour the prepared sauce over the potato mixture. Stir well to ensure even distribution.
06 -
Spread the potato and sauce mixture evenly into the prepared baking dish. Bake at 350°F for 45–50 minutes, until hot and bubbly.
07 -
Line a large baking sheet with two layers of paper towels. Set aside for draining fried onions.
08 -
Add vegetable oil to a large saucepan and heat over high until oil reaches 350°F.
09 -
Working in small groups, carefully lower about 1 cup of sliced onions at a time into the hot oil. Fry, stirring gently, for 2–5 minutes or until onions turn light golden brown.
10 -
Using a slotted spoon or fine mesh strainer, remove fried onions and spread in a single layer on the prepared baking sheet. Immediately sprinkle with kosher salt, tossing gently to coat. Repeat frying and draining with remaining onions, ensuring oil returns to temperature between each batch.
11 -
Once the casserole is baked through, remove from oven. Evenly distribute fried onions over the hot casserole and serve immediately.