Ingredients:

  • 2 kg / 4.4 lbs Starchy potatoes (e.g., Russet, Maris Piper, or King Edward), peeled
  • 1 tsp Fine sea salt (for seasoning layers)
  • ½ tsp Freshly cracked black pepper
  • 4 Tbsp / 56 g Unsalted butter
  • 4 Tbsp / 30 g All-purpose flour (plain flour)
  • 700 ml / 3 cups Full-fat milk
  • 240 ml / 1 cup Heavy cream (double cream)
  • 2 cloves Garlic, finely minced (or grated)
  • ½ tsp Freshly grated nutmeg
  • 1 tsp Fine sea salt (for sauce)
  • ½ tsp White pepper
  • 100 g / 3.5 oz Gruyère or strong cheddar cheese, freshly grated (optional)

Instructions:

  1. Preheat the oven to 180°C / 350°F (160°C Fan/Gas Mark 4). Lightly butter the 9x13 inch baking dish.
  2. Slice the peeled potatoes uniformly thin—ideally 1.5 mm to 2 mm. Place the sliced potatoes in a colander, rinse briefly under cold water to remove excess surface starch, and pat dry thoroughly.
  3. Make the Roux: Melt the butter in a saucepan over medium heat. Whisk in the flour and cook for 1 minute until a smooth paste forms.
  4. Add Dairy: Gradually whisk in the milk, ensuring no lumps remain. Bring the mixture to a gentle simmer, stirring constantly, until the sauce thickens slightly and coats the back of a spoon.
  5. Infuse and Season: Remove the pan from the heat. Whisk in the heavy cream, minced garlic, salt (1 tsp), white pepper, and freshly grated nutmeg. Taste and adjust seasoning. The sauce should taste slightly salty now.
  6. First Layer: Arrange one-third of the sliced, dry potatoes in the bottom of the prepared baking dish in slightly overlapping rows. Season lightly with salt and pepper (from the layering seasoning).
  7. Sauce and Repeat: Pour one-third of the cream sauce evenly over the potato layer. Repeat the layering process (potatoes, salt/pepper, sauce) twice more, finishing with the remaining potatoes and pouring over all the remaining sauce. Gently press down on the layers.
  8. Add Cheese: Sprinkle the grated Gruyère or chosen cheese evenly over the top layer (if using).
  9. Initial Bake (Covered): Cover the dish tightly with aluminum foil. Bake for 45 minutes at 180°C / 350°F.
  10. Final Bake (Uncovered): Remove the foil. Increase the oven temperature slightly to 190°C / 375°F. Bake for an additional 20–30 minutes, or until the top is deeply golden brown and the potatoes are fork-tender throughout.
  11. Rest: Remove the gratin from the oven. Let it rest, uncovered, on a cooling rack for 15 minutes before serving. This allows the starches to settle, preventing the dish from becoming watery.