Ingredients:

  • 0 kg / 4.5 lbs Russet or Yukon Gold Potatoes (peeled and thinly sliced)
  • 1 tsp Kosher Salt (to season layers)
  • 1 clove Garlic (peeled and halved)
  • 1 tbsp / 15g Unsalted Butter (softened, for greasing)
  • 8 tbsp / 115g Unsalted Butter (for sauce)
  • 8 tbsp / 70g All-Purpose Flour
  • 2 Litres / 5 cups Whole Milk (warmed slightly)
  • 1/2 tsp Freshly Ground Black Pepper
  • 1/4 tsp Freshly Grated Nutmeg
  • 1/4 tsp Paprika (optional)
  • 250g / 9 oz Gruyère Cheese (freshly grated)
  • 100g / 3.5 oz Sharp Cheddar Cheese (freshly grated)

Instructions:

  1. Potato Preparation and Dish Setup: Peel the potatoes and slice them uniformly thin (about 1.5 mm/1/16th inch), ideally using a mandoline. Rinse the slices briefly in cold water, then thoroughly pat them dry. Rub the entire interior of a 9x13 inch baking dish with the cut side of the halved garlic clove, then discard the garlic. Grease the dish generously with the softened butter.
  2. Construct the Mornay Sauce: Melt the 8 tbsp of butter in a large saucepan over medium heat. Whisk in the flour until a smooth paste (the roux) forms; cook for 1–2 minutes until slightly nutty, but not brown. Gradually pour in the slightly warmed milk, whisking continuously to prevent lumps. Bring the mixture to a gentle simmer and cook for 5–7 minutes until the sauce has thickened significantly. Remove from heat. Stir in the salt, pepper, nutmeg, and paprika. Add 80% of both the Gruyère and Cheddar (reserving the remainder for the topping) and stir until the cheese is completely melted and the sauce is smooth.
  3. Layering and Assembly: Place a single, slightly overlapping layer of potato slices on the bottom of the prepared dish. Season very lightly with salt. Ladle approximately one-quarter of the Mornay sauce evenly over the potatoes. Continue layering potatoes and sauce, lightly seasoning the potato layers, until all ingredients are used. The final layer should be sauce.
  4. Baking: Preheat oven to 180°C / 350°F (Fan/Convection). Sprinkle the remaining reserved cheese over the top layer. Cover the baking dish tightly with foil and bake for 40 minutes. Remove the foil and increase the oven temperature slightly to 200°C / 400°F. Bake for another 20–30 minutes, or until the top is deeply golden brown, bubbling, and the potatoes are fork-tender. Remove the gratin from the oven and let it rest for at least 15 minutes before slicing and serving. This resting period allows the sauce to set.