Ingredients:

  • 2 lbs (900g) Yukon Gold potatoes, peeled (or left unpeeled)
  • 1 medium yellow onion, thinly sliced (about 1 cup)
  • 4 tablespoons (56g) unsalted butter, divided
  • 2 tablespoons all-purpose flour
  • 2 cups (475ml) whole milk
  • 1 cup (235ml) heavy cream
  • 1 teaspoon kosher salt
  • 1/2 teaspoon black pepper
  • 1/4 teaspoon ground nutmeg
  • 1 cup (100g) shredded Gruyere cheese (or a blend of Gruyere and sharp cheddar) - optional

Instructions:

  1. Peel (optional) and thinly slice potatoes (⅛ inch or 3mm thick). Slice the onion thinly.
  2. Melt 2 tablespoons of butter in a saucepan over medium heat. Whisk in flour and cook for 1-2 minutes. Gradually whisk in milk and cream until smooth. Bring to a simmer, whisking constantly. Reduce heat and simmer gently for 5 minutes, stirring occasionally, until thickened.
  3. Stir in salt, pepper, and nutmeg. Taste and adjust seasoning as needed.
  4. Grease the baking dish with 1 tablespoon of butter. Layer half the potatoes in the dish, overlapping slightly. Top with half the sliced onion and half the cream sauce. Repeat layers with remaining potatoes, onion, and sauce.
  5. Dot the top with the remaining 1 tablespoon of butter, cut into small pieces.
  6. Cover the dish tightly with foil and bake in a preheated oven at 375°F (190°C) for 45 minutes.
  7. Remove the foil and sprinkle with cheese (if using).
  8. Bake uncovered for another 10-15 minutes, or until the potatoes are tender and the top is golden brown and bubbly.
  9. Let stand for 10 minutes before serving.