Ingredients:

  • 3 lbs Russet or Yukon Gold potatoes, peeled and thinly sliced
  • 1 medium yellow onion, thinly sliced
  • 2 cloves garlic, minced
  • 1 tbsp unsalted butter, melted
  • 2 cups low-fat milk or half-and-half
  • 2 tbsp cornstarch
  • 1 tsp kosher salt
  • ½ tsp black pepper
  • 1 ½ cups Gruyère cheese, shredded
  • ½ cup Parmesan cheese, freshly grated
  • 1 tsp fresh thyme leaves

Instructions:

  1. Grease a 9x13 inch ceramic baking dish with melted butter. Use a mandoline to slice potatoes and onions into 1/8-inch rounds. Keep sliced potatoes in a bowl of cold water to prevent oxidation, then pat dry before layering.
  2. In a medium saucepan over medium heat, whisk together the milk (or half-and-half) and cornstarch until smooth. Stir in the salt, pepper, and minced garlic. Bring to a gentle simmer until the sauce thickens enough to coat the back of a spoon.
  3. Remove the sauce from heat and stir in 1 cup of the shredded Gruyère cheese until melted and smooth.
  4. Layer half of the sliced potatoes and onions in the prepared dish, seasoning lightly with salt. Pour half of the cheese sauce evenly over the top.
  5. Repeat the process with the remaining potatoes, onions, and sauce.
  6. Top the dish with the remaining Gruyère, Parmesan cheese, and fresh thyme leaves.
  7. Bake uncovered at 375°F (190°C) for 60–75 minutes until the top is bubbling and deep golden-brown. Let rest for 10 minutes before serving.