Ingredients:

  • 5 lbs (1.1 kg) Yukon Gold or Russet Potatoes, peeled
  • 2 cloves Garlic, minced finely
  • 1 tsp (5 ml) Salt, plus more for seasoning
  • ½ tsp (2.5 ml) Black Pepper
  • 4 Tbsp (55 g) Unsalted Butter (for sauce)
  • ¼ cup (40 g) All-Purpose Flour
  • 2 cups (475 ml) Whole Milk, warmed slightly
  • ½ cup (120 ml) Heavy Cream
  • 1 tsp (5 ml) Fresh Thyme leaves
  • ¼ tsp (1.2 ml) Freshly Grated Nutmeg
  • ½ tsp (2.5 ml) Salt (for sauce)
  • ¼ tsp (1.2 ml) White Pepper
  • 4 oz (115 g) Gruyère Cheese, freshly grated
  • 4 oz (115 g) Sharp Cheddar Cheese, freshly grated
  • 1 Tbsp (15 g) Unsalted Butter, cut into small cubes (for topping)

Instructions:

  1. Preheat the oven to 375°F / 190°C (170°C Fan/Gas Mark 5). Generously butter a 9 x 13 inch baking dish.
  2. Slice the peeled potatoes very thinly (ideally 1/16 to 1/8 inch thick). If using, rinse the slices quickly to remove excess starch, then thoroughly pat them dry. Toss the dried potato slices lightly with minced garlic, 1 tsp salt, and ½ tsp pepper.
  3. Make the Roux: Melt the 4 Tbsp of butter in a large saucepan over medium heat. Whisk in the flour and cook for 1-2 minutes until a nutty smell develops.
  4. Gradually whisk in the warm milk, then the heavy cream, ensuring no lumps form. Bring the mixture to a gentle simmer, whisking constantly until the sauce coats the back of a spoon.
  5. Season the Sauce: Remove the sauce from the heat. Stir in the thyme, nutmeg, salt, and white pepper. Stir in three-quarters (6 oz / 170g) of the grated Gruyère and Cheddar until completely smooth. Taste and adjust seasoning.
  6. Assembly: Pour a thin layer of sauce (about ¼ cup) into the bottom of the prepared baking dish. Arrange one-third of the potato slices in a slightly overlapping pattern over the sauce layer.
  7. Repeat the layering process twice more, ensuring the final potato layer is fully covered by the remaining sauce.
  8. Top the dish with the remaining grated cheese and dot the surface with the small cubes of butter.
  9. Initial Bake: Cover the dish tightly with foil and bake for 35 minutes.
  10. Finish Baking: Remove the foil and bake for an additional 20-30 minutes, or until the top is deeply golden brown and a knife easily pierces the potatoes.
  11. Rest: Remove from the oven and let the dish rest for 10 minutes before slicing and serving to allow the sauce to set properly.