Ingredients:

  • 1.5 kg Potatoes (Half Russet, half Yukon Gold), peeled
  • 1 tbsp (14g) Unsalted butter (for greasing)
  • 60g (4 tbsp) Unsalted butter
  • 35g (1/4 cup) All-purpose flour
  • 3 cloves Garlic, smashed
  • 480ml (2 cups) Whole milk
  • 240ml (1 cup) Heavy cream
  • 1 tsp (5g) Kosher salt
  • 1/2 tsp (2g) Freshly cracked black pepper
  • 2 sprigs Fresh thyme
  • 1/2 tsp (1g) Freshly grated nutmeg
  • 150g (1.5 cups) Gruyère cheese, shredded
  • 30g (1/4 cup) Parmigiano-Reggiano, finely grated

Instructions:

  1. Using a mandoline slicer, cut the peeled potatoes into uniform 1/8-inch (3mm) rounds. Do not soak the slices in water to preserve the surface starch.
  2. In a medium saucepan over medium heat, melt 60g of butter and whisk in the flour for 2 minutes to create a light roux. Slowly whisk in the milk and heavy cream until smooth.
  3. Add the smashed garlic, thyme sprigs, salt, pepper, and nutmeg to the sauce. Simmer until thickened enough to coat the back of a spoon, then strain out the garlic and thyme solids.
  4. Preheat your oven to 350°F (180°C). Grease a 9x13 inch baking dish with 1 tablespoon of butter.
  5. Arrange a third of the potatoes in the dish, lightly season, and pour a portion of the sauce over them. Repeat layers, finishing with the Gruyère and Parmigiano-Reggiano cheese on top.
  6. Cover the dish tightly with aluminum foil and bake for 45 minutes. Remove the foil and bake for an additional 30 minutes until the potatoes are tender and the top is golden brown.