Ingredients:

  • 5 lbs (1.15 kg) Yukon Gold Potatoes, peeled and sliced 1/8 inch thick
  • 1 clove Fresh Garlic, peeled and cut in half
  • 1 Tbsp (15 g) Unsalted Butter, softened (for greasing)
  • Salt and Black Pepper (to season layers)
  • 5 Tbsp (70 g) Unsalted Butter (for sauce)
  • 5 Tbsp (40 g) All-Purpose Flour
  • 3 cups (720 ml) Whole Milk, warmed slightly
  • 1 tsp (2 g) Fresh Thyme Leaves
  • 1/4 tsp (1 g) Freshly Grated Nutmeg
  • 5 tsp (8 g) Kosher Salt (for sauce)
  • 1/2 tsp (2 g) Freshly Ground Black Pepper (for sauce)
  • 6 oz (170 g) Gruyère Cheese, freshly grated
  • 4 oz (115 g) Sharp Cheddar Cheese, freshly grated

Instructions:

  1. Preheat the oven to 375°F (190°C). Generously grease a 9x13 inch baking dish with 1 Tbsp of softened butter. Rub the cut garlic clove vigorously along the bottom and sides of the dish, then discard the garlic. Slice the potatoes uniformly to 1/8 inch (3 mm) thickness, preferably using a mandoline. Do not rinse the slices, as the residual starch helps bind the sauce.
  2. To construct the Mornay sauce, melt the remaining 5 Tbsp butter in a saucepan over medium heat. Whisk in the flour constantly for 1–2 minutes until a smooth, pale roux forms. Gradually whisk in the warm milk, continuing to whisk until the sauce thickens and simmers lightly (about 5 minutes). Reduce heat to low.
  3. Season the Béchamel sauce by whisking in the nutmeg, thyme, Kosher salt, and pepper. Reserve 1/3 of the total grated cheese mix (Gruyère and Cheddar) for the topping. Slowly stir the remaining 2/3 of the cheese into the Béchamel until completely melted and smooth. Remove the sauce from the heat immediately.
  4. Arrange a tight, overlapping layer of sliced potatoes in the prepared baking dish. Season lightly with salt and pepper. Repeat layering the potatoes, seasoning each layer lightly, until all potatoes are used. Carefully pour the prepared Mornay sauce evenly over the potatoes, ensuring the sauce reaches every crevice. Gently press down on the potatoes to submerge them slightly.
  5. Cover the dish tightly with aluminum foil and bake for 45 minutes to steam the potatoes and ensure they become tender. Remove the foil. Sprinkle the remaining reserved cheese mixture evenly over the top. Return to the oven and bake uncovered for another 15–20 minutes, or until the top is deeply golden brown, bubbly, and the potatoes are fork-tender. If the top browns too quickly, tent it loosely with foil.
  6. Remove the Au Gratin Potatoes from the oven and let it rest on a wire rack for 10–15 minutes. This resting time is critical as it allows the sauce to set properly, preventing it from running when sliced. Slice and serve immediately after resting.