Ingredients:

  • 3 lbs Russet or Yukon Gold Potatoes, peeled
  • 2 Tbsp unsalted Butter (for greasing dish)
  • 1 large Garlic clove (for rubbing dish)
  • 4 Tbsp unsalted Butter (for sauce)
  • 4 Tbsp All-Purpose Flour
  • 2 cups Whole or 2% Milk
  • 1 cup Heavy Cream (Double Cream)
  • 1/4 tsp Freshly Grated Nutmeg
  • 1/2 tsp Freshly Ground Black Pepper
  • 3/4 tsp Kosher or Sea Salt
  • 1 cup Gruyère Cheese, freshly grated (divided)
  • 1/2 cup Parmesan Cheese, finely grated
  • 1 tsp Fresh Thyme Leaves (optional)

Instructions:

  1. Preheat oven to 375°F (190°C). Rub the baking dish generously with the cut garlic clove, then grease thoroughly with 2 Tbsp of butter.
  2. Peel and slice potatoes very thinly (ideally 1/8 inch / 3mm thick) using a mandoline or sharp knife. Crucially, do not rinse the starch off the potatoes after slicing.
  3. Make the Roux: In a medium saucepan, melt 4 Tbsp of butter over medium heat. Whisk in the flour and cook gently for 1–2 minutes until a smooth paste forms; do not let it brown.
  4. Build the Sauce: Slowly whisk in the milk until smooth. Bring to a gentle simmer, whisking constantly until the sauce thickens enough to coat the back of a spoon (about 5 minutes).
  5. Finish the Sauce: Remove from heat. Stir in the heavy cream, salt, pepper, and nutmeg. Stir in 1/2 cup of the Gruyère cheese until melted and smooth. Set aside.
  6. Layer the Gratin: Arrange one-third of the sliced potatoes in an even layer in the prepared dish. Sprinkle lightly with salt and pepper. Pour about one-third of the cream sauce evenly over the potatoes.
  7. Repeat the layering process two more times (potatoes, light seasoning, sauce).
  8. Top and Bake (Covered): Sprinkle the remaining 1/2 cup of Gruyère and all of the Parmesan cheese evenly over the top layer. Cover the dish tightly with foil.
  9. Initial Bake: Bake covered for 40 minutes. The potatoes should be mostly tender.
  10. Final Bake (Uncovered): Remove the foil and continue baking for another 20–30 minutes, or until the top is deeply golden brown, bubbly, and the potatoes are completely tender when pierced with a fork.
  11. Rest and Serve: Let the Potatoes Au Gratin rest on the counter for 10 minutes before slicing and serving. This allows the sauce to set up properly.