Ingredients:

  • 8 kg (4 lbs) Yukon Gold or Russet Potatoes, peeled
  • 15 g (1 tbsp) Butter, for greasing
  • 60 g (4 tbsp) Unsalted Butter, for sauce
  • 60 g (1/4 cup) All-Purpose Flour
  • 350 ml (1 1/2 cups) Whole Milk
  • 350 ml (1 1/2 cups) Heavy Cream
  • 10 g (2 tsp) Kosher or Sea Salt, divided
  • 2 g (1/2 tsp) Black Pepper, freshly ground
  • 1 g (1/4 tsp) Nutmeg, freshly grated
  • 225 g (8 oz) Gruyère Cheese, freshly grated
  • 55 g (2 oz) Sharp White Cheddar or Parmesan, for topping

Instructions:

  1. Preheat oven to 190°C (375°F). Lightly grease a 9x13 inch baking dish with butter. Peel the potatoes and slice them uniformly thin (ideally 2–3 mm / 1/8 inch thick). Set aside.
  2. Prepare the Roux: Melt the 60g of butter in a large saucepan over medium heat. Whisk in the flour and cook for 1–2 minutes until a thick, light-blonde paste (roux) forms.
  3. Build the Béchamel: Slowly stream the whole milk and heavy cream into the roux while whisking constantly to prevent lumps. Bring the sauce to a gentle simmer, stirring until it thickens slightly (it should coat the back of a spoon).
  4. Season and Create Mornay: Remove the sauce from the heat. Stir in 1.5 teaspoons of salt, pepper, and nutmeg. Add the 225g of Gruyère cheese and stir until completely melted and smooth. Taste and adjust seasoning.
  5. Layer the Dish: Place one-third of the sliced potatoes evenly across the bottom of the prepared dish. Pour one-third of the cheese sauce over the potatoes, ensuring every slice is moistened. Repeat this layering process two more times, ending with a layer of sauce.
  6. Cover and Initial Bake: Cover the baking dish tightly with aluminum foil. Bake for 45 minutes, allowing the potatoes to steam and soften in the sauce. The potatoes should pierce easily with a fork.
  7. Brown the Gratin: Remove the foil. Sprinkle the remaining 55g of topping cheese over the surface. Return the dish to the oven and bake for 15–20 minutes, or until the top is bubbling, deeply golden brown, and crisp.
  8. Rest and Serve: Allow the Au Gratin Potatoes to rest for 15 minutes before cutting and serving. This allows the sauce to set properly, ensuring clean slices.