Ingredients:

  • 4 kg / 3 lbs King Edward, Maris Piper, or Yukon Gold Potatoes – peeled
  • 1 clove Garlic – peeled and halved
  • 60 g / 4 Tbsp Unsalted Butter
  • 60 g / 1/2 cup All-Purpose (Plain) Flour
  • 700 ml / 3 cups Whole Milk – warmed slightly
  • 240 ml / 1 cup Heavy Cream (Double Cream)
  • 1 tsp Kosher Salt (plus more for layering)
  • 1/2 tsp Freshly Ground Black Pepper
  • 1/4 tsp Freshly Grated Nutmeg
  • 225 g / 8 oz Gruyère Cheese – finely grated (divided: 150g for sauce, 75g for topping)
  • 115 g / 4 oz Sharp Cheddar Cheese – finely grated (for topping)

Instructions:

  1. Prep the Dish: Preheat the oven to 190°C / 375°F / Gas Mark 5. Take the halved garlic clove and thoroughly rub the interior surfaces of the 9x13 inch baking dish; discard the clove. This adds a subtle aroma.
  2. Slice the Potatoes: Using a mandoline set to approximately 2-3mm (1/10 inch), slice the potatoes as thinly and uniformly as possible. Do not rinse them, as the starch helps to thicken the sauce later.
  3. Make the Roux: In a large saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1-2 minutes until pale and sandy (the roux).
  4. Add the Milk: Slowly pour in the warm whole milk, whisking constantly to incorporate the roux and prevent lumps. Bring the sauce to a gentle simmer, stirring until thickened enough to coat the back of a spoon (about 5-7 minutes).
  5. Enrich and Season: Remove the pan from the heat. Stir in the heavy cream, salt, pepper, and nutmeg.
  6. Melt the Cheese: Stir in the 150g portion of Gruyère until fully melted and the sauce is smooth and creamy. Taste and adjust seasoning if necessary.
  7. Layering: Spread a thin layer of the cheese sauce in the bottom of the prepared baking dish.
  8. First Potato Layer: Arrange a layer of potato slices, overlapping them slightly like roof tiles. Season lightly with salt and pepper.
  9. Continue Layering: Pour about a quarter of the remaining sauce over the potatoes, ensuring all slices are coated. Repeat the layering process (Potatoes, Seasoning, Sauce) until all potatoes are used, finishing with a final layer of sauce.
  10. Initial Bake (Covered): Cover the dish tightly with aluminium foil. Bake for 40 minutes. This allows the potatoes to steam and cook fully without burning the top.
  11. Final Bake (Uncovered): Remove the foil. Sprinkle the remaining 75g Gruyère and all the sharp Cheddar evenly over the top.
  12. Achieve Gratin: Return the dish to the oven (uncovered) and bake for another 20-30 minutes, or until the top is deeply golden brown and a knife inserted into the centre meets no resistance.
  13. Crucial Rest: Remove from the oven and let the gratin rest on a rack for at least 10 minutes before serving. This allows the sauce to set properly, preventing it from running all over the plate when sliced.