Ingredients:

  • 4 large Yukon Gold or Russet Potatoes (approx. 1.2 kg), peeled and sliced 1/16 inch (2 mm) thick
  • 1 tsp Unsalted Butter, for greasing the dish
  • Salt (Kosher or Sea), to taste
  • Black Pepper (Freshly Ground), to taste
  • 1 Tbsp Unsalted Butter
  • 1 Tbsp Plain Flour (All-Purpose)
  • 2 large cloves Garlic, finely minced or grated
  • 3 cups Skim Milk (or 1% Milk)
  • 1/2 cup Low-Sodium Chicken Stock
  • 3 sprigs Fresh Thyme (or 1/2 tsp dried)
  • 1 Bay Leaf
  • 1/4 tsp Nutmeg, freshly grated
  • 1/2 cup Aged Sharp Cheddar or Gruyère Cheese, grated
  • 1/4 cup Parmesan Cheese (Parmigiano-Reggiano), shredded, for the top crust

Instructions:

  1. Preheat the oven to 375°F (190°C). Grease a 9 x 13 inch baking dish generously with 1 tsp of butter.
  2. Peel the potatoes and slice them uniformly thin (1/16 inch / 2mm) using a mandoline. Do not rinse the potatoes after slicing, as the starch is needed for thickening.
  3. Melt 1 Tbsp of butter in a medium saucepan over medium heat. Sauté the minced garlic gently for 30 seconds until fragrant—do not let it brown.
  4. Whisk in the plain flour and cook for 1 minute to create a light roux, cooking out the raw flavor.
  5. Slowly whisk in the skim milk and chicken stock until smooth. Add the thyme sprigs, bay leaf, nutmeg, 1 tsp salt, and 1/2 tsp pepper. Bring the mixture just to a simmer, stirring constantly.
  6. Reduce heat and simmer gently for 5 minutes to allow the aromatics to infuse the milk. Remove and discard the bay leaf and thyme sprigs. Keep the sauce warm.
  7. Arrange one-third of the potato slices in a single, slightly overlapping layer across the bottom of the prepared dish. Season lightly with salt and pepper.
  8. Pour one-third of the warm milk mixture over the potatoes. Sprinkle evenly with half of the aged Cheddar/Gruyère cheese.
  9. Repeat the layering process: a second layer of potatoes, seasoning, one-third of the sauce, and the remaining aged Cheddar/Gruyère.
  10. Add the remaining potatoes, season, and pour the final third of the sauce over the top, ensuring the liquid reaches just below the top layer of potatoes.
  11. Cover the dish tightly with foil. Bake for 45 minutes to steam and cook the potatoes through evenly.
  12. Remove the foil. Sprinkle the top evenly with the grated Parmesan cheese. Return the dish to the oven, uncovered, for 15–25 minutes, or until the potatoes are tender and the top is deeply golden brown and bubbling.
  13. Remove the gratin from the oven. Let it rest for 10 minutes before serving. This resting time allows the sauce to set.