Ingredients:

  • 3 lbs Yukon Gold or Russet Potatoes, peeled and thinly sliced (1/8 inch thick)
  • 3 Tbsp unsalted butter, for greasing and roux
  • 3 Tbsp all-purpose flour
  • 2 cups whole milk, warmed
  • 1 cup heavy cream
  • 2 cloves garlic, minced finely
  • 1 tsp fresh thyme leaves
  • 1/4 tsp nutmeg, freshly grated
  • 1 cup packed Gruyère cheese, grated
  • 1/2 cup packed Parmesan cheese, freshly grated
  • 5 tsp salt (to taste)
  • 1/2 tsp black pepper, freshly ground

Instructions:

  1. Preheat oven to 375°F (190°C). Generously butter the 9x13 inch baking dish.
  2. Peel and slice potatoes very thinly (1/8 inch thick). Rinse slices briefly under cold water, then thoroughly dry them with tea towels.
  3. In a bowl, toss the dried potato slices lightly with 1/2 tsp salt and the black pepper.
  4. Melt 3 Tbsp of butter in a medium saucepan over medium heat. Whisk in the flour to create a smooth paste (roux). Cook for 1 minute.
  5. Gradually whisk in the warm milk until smooth. Bring to a gentle simmer, whisking constantly, until the sauce thickens enough to coat the back of a spoon (Béchamel).
  6. Remove from heat. Stir in the heavy cream, minced garlic, thyme, nutmeg, and the remaining salt. Stir in half of the Gruyère and all of the Parmesan cheese until melted and smooth. Taste and adjust seasoning.
  7. Arrange one-third of the seasoned potato slices evenly in the buttered baking dish.
  8. Ladle about one-third of the cream sauce evenly over the potatoes. Repeat the layering process twice more: Potatoes, Sauce.
  9. Top the final layer of potatoes with the remaining Gruyère cheese.
  10. Cover the dish tightly with foil. Bake for 40 minutes at 375°F (190°C).
  11. Remove the foil. Increase oven temperature to 400°F (200°C). Bake uncovered for another 20-30 minutes, or until the top is deep golden brown and the sauce is bubbling thickly.
  12. Crucial Step: Remove from the oven and let the gratin rest for at least 15 minutes before slicing and serving to allow the sauce to fully set.