Ingredients:

  • 4 lbs Yukon Gold Potatoes (or 1.8 kg), peeled and sliced 1/8 inch thick
  • 3 Tbsp Unsalted Butter
  • 3 Tbsp All-Purpose Flour
  • 3 cups Whole Milk, warmed
  • 2 cloves Fresh Garlic, minced
  • 1/4 tsp Nutmeg, freshly grated
  • 8 oz Sharp Mature Cheddar Cheese, freshly grated (divided)
  • 4 oz Gruyère Cheese, freshly grated
  • 1 Tbsp Kosher Salt, divided
  • 1 tsp Black Pepper, freshly ground

Instructions:

  1. Preheat the oven to 375°F (190°C) and generously grease a 9x13 inch baking dish.
  2. Peel the potatoes and use a mandoline to slice them uniformly to 1/8 inch (3 mm) thickness. Pat the slices dry briefly to remove excess moisture; do not rinse.
  3. Gently warm the 3 cups of whole milk in a small saucepan until steaming but not boiling.
  4. Start the Roux: Melt 3 Tbsp of butter in a large saucepan over medium heat. Add the minced garlic and sauté for 30 seconds until fragrant.
  5. Add Flour: Whisk in the 3 Tbsp of flour to form a smooth roux. Cook, stirring continuously, for 1–2 minutes until slightly nutty, but do not let it brown.
  6. Add Milk: Gradually pour the warm milk into the roux, whisking constantly to ensure no lumps form. Bring the mixture to a gentle simmer.
  7. Thicken: Cook for 5–7 minutes, stirring until the sauce is thick enough to coat the back of a spoon (Béchamel stage). Remove from heat.
  8. Add Cheese & Seasoning: Stir in 6 oz of the grated Sharp Cheddar and all the Gruyère (reserving the final 2 oz of Cheddar for the topping). Stir until the cheese is melted and the sauce is smooth (Mornay sauce). Season heavily with salt, pepper, and freshly grated nutmeg. Taste and adjust seasoning.
  9. Assembly: Pour about one-third of the Mornay sauce onto the bottom of the prepared baking dish and spread evenly.
  10. Potato Layers: Arrange half of the sliced potatoes tightly overlapping in the dish. Season generously with salt and pepper.
  11. Mid-Layer: Pour half of the remaining sauce over the potatoes, ensuring all slices are coated.
  12. Final Layer: Layer the remaining potatoes, overlapping them again. Season with salt and pepper.
  13. Final Sauce: Pour the final portion of the Mornay sauce over the top layer. Tap the dish lightly to help the sauce settle between the slices.
  14. Topping: Sprinkle the reserved 2 oz of grated Cheddar evenly over the surface.
  15. Initial Bake (Covered): Cover the dish tightly with foil. Bake for 45 minutes at 375°F (190°C).
  16. Final Bake (Uncovered): Remove the foil and continue baking for 25–30 minutes, or until the potatoes are tender (test with a sharp knife) and the top is deeply golden brown and bubbling.
  17. Rest: Remove from the oven and let the gratin rest for 10 minutes before slicing and serving. This allows the creamy sauce to set properly.