Ingredients:

  • 3 lbs Yukon Gold potatoes, peeled and sliced into 1/8-inch rounds
  • 1 medium yellow onion, very thinly sliced
  • 1/4 cup unsalted butter
  • 1/4 cup all-purpose flour
  • 3 cloves garlic, minced
  • 2 cups whole milk, room temperature
  • 1 cup heavy whipping cream
  • 1.5 teaspoons salt
  • 1/2 teaspoon freshly cracked black pepper
  • 1 teaspoon fresh thyme leaves
  • 3 cups sharp cheddar cheese, shredded and divided
  • 1/2 cup freshly grated Parmesan cheese

Instructions:

  1. Prepare the potatoes by slicing them into uniform 1/8-inch rounds. Keep them in a bowl of cold water to prevent browning, then pat completely dry before assembly.
  2. In a large saucepan, melt the butter over medium heat. Sauté the sliced onions until translucent, then add the minced garlic and cook for 30 seconds until fragrant.
  3. Whisk in the flour and cook for 1-2 minutes to remove the raw flour taste, ensuring the roux does not brown.
  4. Slowly pour in the milk and heavy cream while whisking constantly. Simmer the mixture until the sauce thickens enough to coat the back of a spoon.
  5. Remove the saucepan from heat and stir in 2.5 cups of the shredded Cheddar and all the Parmesan until the sauce is smooth. Season with salt, pepper, and thyme.
  6. Grease a 9x13 inch baking dish. Arrange half of the potatoes in an even layer and pour half of the cheese sauce over them. Repeat with the remaining potatoes and sauce.
  7. Cover the dish tightly with aluminum foil and bake at 375°F (190°C) for 45 minutes.
  8. Remove the foil and sprinkle the reserved 1/2 cup of Cheddar cheese on top. Bake for another 20-25 minutes until the top is golden brown and the sauce is bubbling.
  9. Allow the dish to rest for 10-15 minutes before serving to let the sauce thicken and set.