Ingredients:

  • 3 medium Sweet Potatoes (approx. 3.5 lbs), peeled and sliced 1/8 inch (3 mm) thick
  • 2 Tablespoons Unsalted Butter, melted, plus extra for greasing
  • ½ cup Heavy Cream (Double Cream), full fat
  • ½ cup Whole Milk
  • 2 cloves Garlic, minced very finely
  • 1 Tablespoon Fresh Thyme Leaves, stripped from the stem
  • 1 teaspoon Fresh Sage, finely chopped
  • ¼ teaspoon Nutmeg, freshly grated
  • 1 teaspoon Kosher Salt, plus more for layering
  • ½ teaspoon Black Pepper, freshly ground
  • 5 oz Gruyère Cheese, finely grated
  • 2 Tablespoons Parmesan Cheese, finely grated

Instructions:

  1. Preheat oven to 375°F (190°C). Generously grease the baking dish (10-inch round or 9x9 inch square) with melted butter.
  2. Peel the sweet potatoes. Using a mandoline, slice them to a uniform thickness of 1/8 inch (3 mm). Place the slices in a large bowl.
  3. Prepare Cream Base: In a small saucepan, gently warm the heavy cream and milk (do not boil). Stir in the minced garlic, thyme, sage, nutmeg, 1 teaspoon salt, and pepper. Remove from heat.
  4. Toss Potatoes: Pour the remaining 2 tablespoons of melted butter over the sliced sweet potatoes. Toss lightly to coat.
  5. First Layer: Arrange the first layer of sweet potato slices vertically and tightly packed, overlapping them slightly, starting from the outside edge of the baking dish and working your way inward to create a decorative pattern.
  6. Season and Repeat: Once the dish base is covered, sprinkle lightly with extra salt and pepper. Repeat the layering process until all the sweet potatoes are used, packing them tightly.
  7. Add Liquid: Slowly and evenly pour the warm cream mixture over the entire dish, ensuring the liquid permeates the layers. The liquid should reach about 3/4 of the way up the sides.
  8. Add Cheese Topping: Combine the grated Gruyère and Parmesan. Sprinkle about two-thirds of the cheese mixture evenly over the top layer.
  9. Initial Bake (Covered): Cover the dish tightly with aluminium foil. Bake for 40 minutes at 375°F (190°C). This steaming ensures the potatoes become tender.
  10. Final Bake (Uncovered): Remove the foil. Sprinkle the remaining cheese mixture over the top. Return the Tian to the oven and bake for another 15–20 minutes, or until the top is deeply golden brown and the potatoes are easily pierced with a fork.
  11. Rest & Serve: Remove the Tian from the oven. Let it rest on the counter for 10 minutes before slicing. This crucial step allows the cream and starches to set up.