Ingredients:

  • 4 lbs (1.8 kg) Russet or Yukon Gold Potatoes, peeled and uniformly sliced (1/8 inch / 3mm thick)
  • 1 tsp Kosher Salt
  • 1/2 tsp Freshly Ground Black Pepper
  • 4 Tbsp (56 g) Unsalted Butter
  • 4 Tbsp (30 g) All-Purpose Flour (AP)
  • 3 cups (710 ml) Whole Milk, warmed
  • 1 cup (240 ml) Heavy Cream
  • 1/2 tsp Garlic Powder
  • 1/4 tsp Nutmeg, freshly grated
  • 6 oz (170 g) Gruyère Cheese, shredded (4 oz for sauce, 2 oz for topping)
  • 4 oz (115 g) Sharp White Cheddar, shredded

Instructions:

  1. Preheat oven to 375°F (190°C). Grease the 9x13 inch baking dish. Slice potatoes uniformly to 1/8 inch thickness; rinse the slices thoroughly in cold water to remove excess starch, then dry well. Season lightly with salt and pepper.
  2. Establish the Roux: In a large saucepan, melt the butter over medium heat. Whisk in the flour to form a smooth paste (roux). Cook for 1-2 minutes until slightly bubbly and pale blonde.
  3. Create the Béchamel: Gradually whisk in the warm whole milk until the mixture is smooth and thickened. Slowly add the heavy cream, continuing to whisk until the sauce coats the back of a spoon. Bring to a gentle simmer.
  4. Finish the Cheese Sauce: Remove the sauce from the heat. Stir in the garlic powder, nutmeg, 1 teaspoon of salt, 4 oz (115g) of Gruyère, and all the sharp cheddar until the cheese is completely melted and the sauce is velvety smooth.
  5. Assemble the Gratin: In a large bowl, gently toss the dried potato slices with about 3/4 of the prepared cheese sauce, ensuring every slice is coated.
  6. Layer and Top: Arrange the coated potato slices vertically or shingled in the prepared baking dish. Pour any remaining sauce evenly over the top. Sprinkle the remaining 2 oz (55g) of Gruyère cheese over the surface.
  7. Bake (Covered): Cover the dish tightly with aluminum foil and bake for 45 minutes until the potatoes are tender when pierced.
  8. Bake (Uncovered & Finish): Remove the foil. Increase the oven temperature to 400°F (200°C) and continue baking for 15-20 minutes until the top is deeply golden brown and bubbly.
  9. Rest and Serve: Allow the gratin to rest for 15 minutes before serving. This allows the sauce to set properly.