Ingredients:

  • 5 kg (3.3 lbs) Yukon Gold potatoes, peeled
  • 1 tsp Kosher Salt, plus extra for seasoning
  • 1/2 tsp Freshly ground black pepper
  • 4 Tbsp (56 g) Unsalted Butter (for the roux)
  • 4 Tbsp (30 g) All-Purpose Flour
  • 2 cloves Garlic, minced fine
  • 2 cups (500 ml) Whole Milk
  • 1 cup (250 ml) Heavy Cream
  • 1 tsp Fresh Thyme leaves
  • Generous Pinch Freshly grated Nutmeg (~0.5 g)
  • 1 tsp Fine Sea Salt
  • 1/4 tsp White Pepper
  • 7 oz (200 g) Gruyère Cheese, freshly grated (divided)
  • 2 oz (50 g) Parmesan Cheese, freshly grated (divided)
  • 1 Tbsp Unsalted Butter, melted (optional, for finishing)

Instructions:

  1. Preheat the oven to 375°F (190°C). Lightly butter a 9 x 13 inch (23 x 33 cm) baking dish.
  2. Peel the potatoes. Using a mandoline or sharp knife, slice the potatoes into uniform 1/8 inch (3 mm) rounds. Pat the slices dry to remove excess moisture. Toss the potatoes lightly with 1 tsp of Kosher salt and pepper.
  3. Start the Roux: Melt the 4 Tbsp of butter in a heavy-bottomed saucepan over medium heat. Whisk in the flour to create a smooth roux. Cook for 1 minute, stirring constantly.
  4. Infuse Flavor: Add the minced garlic and thyme to the roux and cook for 30 seconds until fragrant.
  5. Add Dairy: Slowly whisk in the cold milk and cream mixture until fully incorporated, preventing lumps. Continue whisking until the sauce comes to a gentle simmer and visibly thickens (it should coat the back of a spoon).
  6. Season and Add Cheese: Remove the sauce from the heat. Stir in the salt, white pepper, and nutmeg. Add three-quarters of the Gruyère and half of the Parmesan cheese. Stir until the cheese is fully melted and the sauce is smooth.
  7. Pour a thin layer of Béchamel sauce onto the bottom of the prepared baking dish—just enough to cover the base.
  8. Layer half of the potato slices, overlapping them slightly. Pour half of the remaining sauce over the potatoes. Repeat with the remaining potato slices and the remaining sauce. Gently press down on the layers.
  9. Initial Bake (Covered): Cover the dish tightly with foil. Bake for 50 minutes at 375°F (190°C).
  10. Uncover and Brown: Remove the foil. Sprinkle the remaining Gruyère and Parmesan cheese over the top.
  11. Final Bake (Uncovered): Return to the oven for 20–25 minutes, or until the top is bubbling, deeply golden brown, and the potatoes are tender when pierced with a fork.
  12. Rest: Remove the gratin from the oven and let it rest on a wire rack for at least 15 minutes before serving. This allows the sauce to set and ensures clean servings.