Ingredients:

  • lbs Russet or Yukon Gold Potatoes, peeled and very thinly sliced (1/16 inch)
  • Tbsp Unsalted Butter
  • Tbsp All-Purpose Flour
  • cups Whole Milk (warmed)
  • cup Heavy Cream (warmed)
  • oz Freshly Grated Gruyère Cheese, divided
  • /4 tsp Freshly Grated Nutmeg
  • clove Garlic, finely minced
  • tsp Fresh Thyme Leaves
  • tsp Kosher Salt (or to taste)
  • /2 tsp Freshly Ground Black Pepper (or to taste)
  • Tbsp Unsalted Butter (for greasing)
  • /4 cup Panko Breadcrumbs (Optional)

Instructions:

  1. Preheat oven to 375°F (190°C). Generously butter the 9x13 inch baking dish. Peel and uniformly slice potatoes (aim for paper-thin thickness using a mandoline or sharp knife).
  2. In a small saucepan, gently warm the milk, cream, minced garlic, thyme, salt, and pepper. Do not boil; let steep for 10 minutes, then strain out the garlic and thyme.
  3. In a separate saucepan over medium heat, melt the 4 Tbsp butter. Whisk in the flour and cook for 2 minutes to create a pale blond roux.
  4. Gradually whisk the warm, infused milk mixture into the roux until smooth. Bring to a gentle simmer, stirring constantly, until the sauce thickens enough to coat the back of a spoon (Béchamel).
  5. Remove the sauce from heat. Stir in the nutmeg and half (3 oz) of the Gruyère cheese until melted and smooth. Taste and adjust seasoning.
  6. Begin assembly: Arrange one-third of the potato slices in an even layer in the prepared dish. Season lightly with salt and pepper. Ladle one-third of the cheese sauce evenly over the potatoes.
  7. Repeat the layering process twice more (Potatoes, Seasoning, Sauce). Top the final layer of sauce with the remaining Gruyère cheese and the optional Panko breadcrumbs.
  8. Cover the dish tightly with foil and bake for 60 minutes, or until the potatoes are tender when pierced with a skewer.
  9. Remove the foil. Increase oven temperature to 400°F (200°C) if necessary, and bake uncovered for 15–20 minutes until the top is deeply golden brown and bubbly.
  10. Crucially, let the gratin rest on the counter for at least 15 minutes before slicing and serving. This allows the sauce to set properly.