Ingredients:

  • 2.5 lbs Yukon Gold potatoes, peeled
  • 1 medium yellow onion, thinly sliced
  • 2 cloves garlic, minced
  • 2 tablespoons fresh thyme leaves, chopped
  • Salt and freshly ground black pepper, to taste
  • 2 tablespoons unsalted butter, for greasing the dish
  • 4 tablespoons unsalted butter
  • 4 tablespoons all-purpose flour
  • 3 cups whole milk
  • 1/2 cup Gruyere cheese, grated, plus extra for topping
  • 1/4 teaspoon nutmeg, freshly grated
  • Salt and freshly ground white pepper, to taste

Instructions:

  1. Preheat oven to 375°F (190°C/Gas Mark 5). Grease a 9x13 inch baking dish with butter. Thinly slice the potatoes using a mandoline or knife. Place in a bowl of cold water to prevent browning. Drain well and pat dry before layering.
  2. In a large saucepan, gently sauté the onion in butter until softened and translucent, about 5-7 minutes. Add the minced garlic and thyme and cook for another minute until fragrant.
  3. Melt the butter in the saucepan over medium heat. Whisk in the flour and cook for 1-2 minutes, creating a roux. Gradually whisk in the milk, a little at a time, ensuring each addition is fully incorporated before adding more. Continue whisking until the sauce thickens and is smooth, about 5-7 minutes.
  4. Remove the saucepan from the heat. Stir in the grated Gruyere (or cheddar) until melted and smooth. Season with nutmeg, salt, and white pepper to taste.
  5. Layer one-third of the potato slices in the prepared baking dish, slightly overlapping. Season with salt and pepper. Top with half of the onion mixture and one-third of the béchamel sauce. Repeat layers. Finish with a final layer of potatoes, remaining béchamel sauce, and a sprinkle of extra grated cheese.
  6. Cover the baking dish tightly with foil and bake for 45 minutes. Remove the foil and bake for another 10-20 minutes, or until the potatoes are tender and the top is golden brown and bubbly. Test for doneness with a fork.
  7. Let the scalloped potatoes rest for at least 10 minutes before serving.