Ingredients:

  • 6 large (about 4 lbs / 1.8 kg) Yukon Gold Potatoes, peeled
  • 2 Tbsp (30g) Unsalted Butter, softened (for greasing the dish)
  • 1/2 tsp (2.5g) Kosher Salt (plus more for seasoning the layers)
  • 1 cup (240 ml) Whole Milk (3.5% fat)
  • 2 cups (480 ml) Heavy Cream (Double Cream, 35%+ fat)
  • 2 cloves Garlic, smashed but left whole
  • 4 sprigs Fresh Thyme
  • 1/4 tsp Freshly Grated Nutmeg
  • 1/4 tsp White Pepper
  • 4 Tbsp (56g) Unsalted Butter
  • 4 Tbsp (30g) All-Purpose Flour (Plain Flour)
  • 6 oz (170g) Gruyère Cheese, freshly grated
  • 2 oz (55g) Parmesan Cheese, freshly grated (for the crust)

Instructions:

  1. Prepare Potatoes: Peel the potatoes. Using a mandoline or very sharp knife, slice them uniformly thin (1/16 to 1/8 inch / 1.5–2 mm is ideal). Consistency is key for even cooking.
  2. Rinse and Dry: Rinse the sliced potatoes briefly in cold water to remove excess starch. Dry them thoroughly using a clean tea towel or kitchen paper. Set aside.
  3. Infuse Dairy: In a medium saucepan, combine the whole milk, heavy cream, garlic cloves, thyme sprigs, nutmeg, and white pepper. Bring the mixture to a gentle simmer over medium heat.
  4. Steep: Immediately remove from heat, cover, and let the mixture steep for 10 minutes to allow the aromatics to infuse the cream. Strain the cream mixture through a fine-mesh sieve into a jug, discarding the solids. Keep warm.
  5. Make the Roux: In the same (now empty) saucepan, melt the 4 Tbsp of butter over medium heat. Whisk in the flour and cook, stirring constantly, for 1–2 minutes until a pale paste (roux) forms. Do not brown the roux.
  6. Add Dairy: Slowly and gradually whisk the warm, infused cream mixture into the roux. Start with a small amount to make a smooth paste, then add the rest.
  7. Thicken and Season: Bring the sauce to a gentle simmer, stirring constantly (about 3–5 minutes), until the sauce coats the back of a spoon. Remove from heat and stir in the grated Gruyère cheese until fully melted and smooth. Taste and season heavily with Kosher salt—the sauce must carry the flavor for the potatoes.
  8. Preheat & Grease: Preheat the oven to 375°F (190°C). Generously grease the 9x13 inch baking dish with softened butter.
  9. Layer the Gratin: Pour a thin layer of sauce (about 1/2 cup) over the bottom of the prepared dish.
  10. Potato Layers: Arrange one-third of the potato slices in an overlapping, slightly fanned pattern over the sauce. Gently pour half of the remaining sauce over the potatoes, ensuring even distribution. Lightly sprinkle the layer with salt.
  11. Repeat: Add the next one-third of the potatoes, pour the remaining sauce over the top, and sprinkle with a touch more salt. Finish with the final layer of potatoes, pressing down gently. The top layer of potatoes should be mostly covered by the sauce, but the tips should poke out.
  12. Initial Bake (Covered): Cover the dish tightly with foil. Bake for 45 minutes. This steams the potatoes and ensures they cook through evenly.
  13. Second Bake (Uncovered): Remove the foil. Sprinkle the top evenly with the grated Parmesan cheese. Return to the oven and bake uncovered for another 25–35 minutes, or until the top is deeply golden brown. Test for doneness by piercing the centre with a knife; the knife should slide through easily.
  14. Crucial Rest: Remove the gratin from the oven and let it rest on a wire rack for a minimum of 15 minutes before serving. This allows the sauce to set and the layers to hold their structure.