Ingredients:

  • 5 lbs Sweet Potatoes, large, firm, peeled
  • 5 tsp Kosher Salt, plus extra for seasoning layers
  • 1/2 tsp Freshly Ground Black Pepper
  • 8 Tbsp (1 stick) Unsalted Butter, divided (4 Tbsp for browning, 4 Tbsp softened)
  • 2 cups Heavy Cream (35% or higher)
  • 1 cup Whole Milk
  • 12 large Fresh Sage Leaves, finely chopped
  • 1 Tbsp Fresh Thyme Leaves (leaves only)
  • 1/2 tsp Fresh Nutmeg, freshly grated
  • 1/2 cup Panko Breadcrumbs
  • 1/2 cup Gruyère Cheese, freshly grated (or sharp white cheddar)
  • 4 Tbsp Softened Butter (for coating the topping)

Instructions:

  1. Prep the Potatoes: Peel the sweet potatoes. Using a mandoline or sharp knife, slice the potatoes into uniform discs, approximately 1/8 inch (3 mm) thick. Place slices immediately into a bowl of cold water.
  2. Brown the Butter: Melt 4 Tbsp of unsalted butter in a medium saucepan over medium heat. Continue heating, swirling occasionally, until the milk solids turn golden-brown and smell nutty (about 5–7 minutes). Remove from heat immediately and pour into a heatproof bowl to stop cooking.
  3. Infuse the Cream: In the same saucepan, combine the heavy cream, whole milk, chopped sage, thyme, kosher salt, pepper, nutmeg, and the browned butter. Whisk well. Bring the mixture just to a simmer over medium-low heat, then immediately remove from the heat.
  4. Assemble the Gratin: Drain the potato slices thoroughly and pat them dry. Lightly grease a 9x13 baking dish with the remaining 4 Tbsp softened butter. Lay the potato slices in overlapping rows, covering the base of the dish. Pour a generous ladle of the cream mixture over the potatoes. Repeat the layering process until all potatoes are used, ensuring each layer is seasoned lightly with salt and pepper. Pour the remaining cream mixture evenly over the top. The top layer should be mostly submerged.
  5. Chill (Crucial Make-Ahead Step): Cover the dish tightly with foil or cling film. Place in the refrigerator and chill for a minimum of 4 hours, and ideally overnight (up to 24 hours). This allows the potatoes to absorb the cream and flavorings.
  6. Preheat and First Bake: Preheat your oven to 375°F (190°C). Remove the gratin from the fridge 30 minutes before baking to take the chill off. Place the foil-covered dish in the preheated oven and bake for 40 minutes.
  7. Prepare the Topping: While the gratin bakes, combine the Panko breadcrumbs, grated Gruyère, and the remaining 4 Tbsp of softened butter in a small bowl. Use your fingers to mix until the breadcrumbs are coated.
  8. Second Bake (Uncovered): Remove the gratin from the oven. Carefully remove the foil. Sprinkle the breadcrumb/cheese mixture evenly over the top.
  9. Finish: Return the dish to the oven and bake for another 20–30 minutes, or until the top is golden brown, bubbling fiercely, and a knife inserted into the centre meets no resistance.
  10. Rest and Serve: Allow the gratin to rest for 10–15 minutes before serving. This helps the sauce set and makes it easier to serve.