Ingredients:

  • 5 kg (3 lbs) Frozen Diced Potatoes or Shredded Hash Browns (do not thaw)
  • 60 g (4 tbsp / 1/4 cup) Unsalted Butter
  • 60 g (4 tbsp / 1/4 cup) All-Purpose (Plain) Flour
  • 475 ml (2 cups) Whole Milk (lukewarm)
  • 240 g (1 cup) Full-Fat Sour Cream or Crème Fraîche
  • 450 g (4 cups packed) Mature Cheddar Cheese, freshly grated (reserve 1 cup for the topping)
  • 1 tsp Fine Sea Salt
  • 1/2 tsp Freshly Ground Black Pepper
  • 1 tsp Onion Powder
  • 1/2 tsp Garlic Powder
  • 1/2 tsp Dry Mustard Powder
  • 60 g (4 tbsp) Unsalted Butter, melted (for topping)
  • 120 g (1.5 cups) Panko Breadcrumbs, or Crushed Cornflakes

Instructions:

  1. Preheat the oven to 180°C / 350°F and lightly grease the 9x13 inch baking dish. Place the frozen potatoes into a very large mixing bowl.
  2. Make the Roux: Melt the butter (60g) in a saucepan over medium heat. Whisk in the flour (60g) and cook gently for 1–2 minutes, stirring constantly to remove the raw flour taste.
  3. Create the Sauce Base: Gradually whisk in the lukewarm milk until the sauce is smooth and thickens slightly (like double cream). Reduce heat to low.
  4. Remove the pan from the heat. Stir in 3 cups of the grated cheddar, the sour cream, salt, pepper, onion powder, garlic powder, and dry mustard.
  5. Stir until the cheese is fully melted and the sauce is uniform and silky. Taste and adjust seasoning if necessary.
  6. Pour the completed cheese sauce immediately over the frozen potatoes in the mixing bowl.
  7. Use a large spoon or spatula to gently fold and mix until all potatoes are thoroughly coated in the sauce.
  8. Scrape the potato mixture evenly into the prepared baking dish.
  9. Prepare Topping: In a small bowl, combine the 60g melted butter, panko breadcrumbs (or crushed cornflakes), and the reserved 1 cup of cheddar cheese. Mix well.
  10. Sprinkle the cheese and crumb mixture evenly over the top of the potatoes.
  11. Bake: Cover the dish tightly with foil and bake for 30 minutes. Remove the foil, increase the oven temperature slightly to 190°C / 375°F, and bake for another 10–15 minutes until the topping is deeply golden brown and the edges are bubbling furiously.
  12. Rest: Allow the casserole to rest on a wire rack for 10 minutes before serving. This crucial step allows the sauce to set properly.