🔹 Prep the Casserole
Preheat oven to 350°F (175°C). Grease a 9×13-inch baking dish with cooking spray.
Thaw hash browns completely and pat dry with paper towels to remove excess moisture.
🔹 Make the Potato Base
In a large bowl, mix hash browns, melted butter, shredded cheddar, and sour cream until well combined. Set aside.
🔹 Prepare the Creamy Sauce
In a skillet over medium heat, melt butter until foamy.
Add diced onion and sauté until soft and translucent (3–5 minutes).
Stir in garlic and cook for 1 minute.
Sprinkle flour, salt, pepper, cayenne, and mustard powder over the mixture. Stir constantly to form a roux.
Gradually whisk in chicken broth and milk until smooth.
Cook until thickened to a gravy-like consistency (2–3 minutes).
Pour sauce over potato mixture and fold gently to coat evenly.
🔹 Bake
Spread mixture evenly in prepared dish.
Bake uncovered for 45–50 minutes, until bubbly and heated through.
🔹 Make the Crispy Onion Topping
Line a baking sheet with paper towels.
Heat oil in a heavy-bottomed pot to 350°F.
Fry onion rings in batches (about 1 cup at a time) for 2–5 minutes, stirring occasionally until golden brown.
Transfer to paper towels and sprinkle with salt while hot. Repeat with remaining onions, letting oil return to temperature between batches.
🔹 Assemble & Serve
Once casserole is baked, top immediately with crispy onions.
Serve hot while the topping is still crunchy.
🍽️ Tips & Variations
Make Ahead: Assemble the casserole (without topping) and refrigerate up to 24 hours before baking.
Reheating: Warm leftovers in the oven and re-crisp onions separately if needed.
Dietary Swaps: Use Greek yogurt instead of sour cream, or gluten-free flour for the roux.
Shortcut Version: Use store-bought French fried onions for a faster topping.
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