Ingredients:

  • 3 lbs Russet or Yukon Gold potatoes, peeled and sliced 1/8 inch thick
  • 1 clove garlic, peeled and halved
  • 4 tbsp unsalted butter
  • 1/4 cup all-purpose flour
  • 2.5 cups whole milk, warmed
  • 1 cup heavy cream
  • 8 oz sharp cheddar cheese, freshly shredded
  • 4 oz Gruyère cheese, freshly shredded
  • 1 tsp kosher salt
  • 1/2 tsp freshly cracked black pepper
  • 1/2 tsp dry mustard powder
  • 1/4 tsp freshly grated nutmeg
  • 1 tsp fresh thyme leaves

Instructions:

  1. Preheat your oven to 375°F (190°C). Rub the inside of a 9x13 inch baking dish vigorously with the cut side of the garlic clove, then grease generously with butter.
  2. In a heavy-bottomed saucepan, melt 4 tablespoons of butter over medium heat. Whisk in the flour and cook for 2 minutes to create a pale roux. Slowly stream in the warm milk and heavy cream, whisking constantly until the sauce thickens and coats the back of a spoon.
  3. Remove the sauce from heat. Fold in 6 oz of the shredded cheddar and all the Gruyère, salt, pepper, dry mustard, and nutmeg. Stir until the cheese is melted and the sauce is silky.
  4. Arrange half of the sliced potatoes in the prepared dish in overlapping rows. Pour half of the cheese sauce evenly over the potatoes. Repeat with the remaining potatoes and the rest of the sauce, pressing down gently with a spatula to ensure the sauce migrates between layers.
  5. Cover the dish tightly with aluminum foil. Bake for 30 minutes. Remove the foil, sprinkle with the remaining 2 oz of cheddar and thyme, and bake for an additional 45 minutes until the potatoes are tender and the top is a deep mahogany brown.