Ingredients:

  • 5 kg (3.3 lb) Yukon Gold or Russet Potatoes, peeled
  • 15 ml (1 Tbsp) Unsalted Butter, for greasing
  • 60 g (4 Tbsp) Unsalted Butter (for roux)
  • 60 g (1/2 cup) All-Purpose Flour
  • 700 ml (3 cups) Whole Milk, warmed
  • 5 ml (1 tsp) Salt (or to taste)
  • 1 ml (1/4 tsp) Ground White Pepper
  • 1 ml (1/4 tsp) Freshly Grated Nutmeg
  • 5 g (1 tsp) Garlic Powder
  • 150 g (1.5 cups) Gruyère Cheese, shredded
  • 100 g (1 cup) Sharp White Cheddar, shredded

Instructions:

  1. Preheat oven to 190°C (375°F). Generously butter the 9x13 inch baking dish. Slice the potatoes extremely thinly (ideally 3mm / 1/8 inch) using a mandoline. Rinse the slices briefly in cold water to remove excess starch, then pat them thoroughly dry.
  2. Construct the Roux: In the saucepan, melt the 60g butter over medium heat. Whisk in the flour and cook for 1-2 minutes until the mixture (roux) is smooth and smells slightly nutty; do not brown it.
  3. Create the Béchamel: Slowly pour in the warm milk while continuously whisking to prevent lumps. Bring the sauce to a gentle simmer, cooking until it thickens sufficiently to coat the back of a spoon (about 5-7 minutes).
  4. Flavor and Melt: Remove the Béchamel from the heat. Stir in the salt, pepper, nutmeg, and garlic powder. Incorporate about three-quarters (75%) of the shredded Gruyère and Cheddar until the cheese is fully melted and the sauce is uniform and rich.
  5. Layer the Dish: Dip the dried potato slices into the warm sauce mixture, ensuring each slice is coated. Arrange half of the coated potatoes tightly in the prepared baking dish, overlapping slightly. Pour half of the remaining sauce evenly over the first layer.
  6. Complete the Layers: Arrange the remaining coated potato slices over the first layer. Pour the remaining sauce over the top, tilting the dish gently to ensure the sauce seeps into all crevices.
  7. Final Bake (Covered): Cover the dish tightly with aluminum foil. Bake for 30 minutes to ensure the potatoes become fork-tender.
  8. Gratin Finish (Uncovered): Remove the foil. Sprinkle the remaining 25% of the cheese evenly over the top. Return to the oven and bake for another 25-30 minutes, or until the top is deeply golden brown and the sauce is vigorously bubbling.
  9. Rest: Remove the dish from the oven and let it rest on a rack for at least 15-20 minutes before serving. This allows the sauce to set and prevents a watery casserole.