Ingredients:

  • 4 large (about 3 lbs) Yukon Gold Potatoes, peeled and sliced 1/8 inch thick
  • 4 Tbsp Unsalted Butter
  • 4 Tbsp All-Purpose Flour
  • 3 cups Whole Milk (cold)
  • 1 small Garlic clove, finely minced or grated
  • 1 tsp Kosher Salt
  • 1/2 tsp Freshly Ground Black Pepper
  • 1/4 tsp Freshly Grated Nutmeg
  • 8 oz Gruyère Cheese, freshly grated (divided)
  • 8 oz Sharp Cheddar Cheese, freshly grated (divided)

Instructions:

  1. Potato Preparation (Consistency is Key): Peel and uniformly slice the potatoes into thin rounds (ideally 1/8 inch / 3mm). Rinse the slices in a large bowl of cold water for 5 minutes, drain thoroughly, and pat completely dry. Preheat oven to 400°F (200°C). Grease a 9x13 inch baking dish generously with butter.
  2. Construct the Mornay Sauce Base: Melt the 4 Tbsp of butter in a medium saucepan over medium heat. Whisk in the flour until a smooth paste (roux) forms. Cook for 1–2 minutes until pale golden and smelling slightly nutty. Gradually whisk in the cold milk until fully incorporated. Add the minced garlic. Bring the mixture to a gentle simmer, whisking frequently, until the sauce thickens enough to coat the back of a spoon (about 5–7 minutes). Do not boil aggressively. Remove the sauce from the heat. Stir in the salt, black pepper, and vital nutmeg.
  3. Finish the Sauce and Assemble: Add 8 oz (4 oz Gruyère, 4 oz Cheddar) of grated cheese to the warm sauce base. Stir until completely melted and smooth. Spread a very thin layer of sauce across the bottom of the prepared baking dish. Arrange one-third of the dried potato slices in a slightly overlapping pattern over the sauce layer. Pour one-third of the remaining sauce evenly over the potatoes. Repeat this layering process until all potatoes and sauce are used, finishing with a layer of sauce. Sprinkle the remaining 2 oz (1 oz Gruyère, 1 oz Cheddar) of reserved cheese evenly over the top layer.
  4. Baking and Resting: Cover the dish tightly with foil. Bake at 400°F (200°C) for 30 minutes. Remove the foil. Reduce the oven temperature to 375°F (190°C). Bake for another 25–30 minutes, or until the top is deeply golden brown and the potatoes are fork-tender. Remove from the oven and allow the gratin to rest for 10 minutes before serving.