Ingredients:

  • 5 lbs (1.1 kg) Yukon Gold or Russet Potatoes, peeled and thinly sliced
  • 1 tsp Fine Sea Salt (for seasoning potatoes)
  • 4 Tbsp Unsalted Butter (for the roux)
  • 4 Tbsp All-Purpose Flour
  • 3 cups (710 ml) Whole Milk, room temperature
  • 1 cup (240 ml) Chicken Stock (low sodium)
  • 5 cups (180 g) Sharp Cheddar Cheese, freshly grated
  • 1 cup (120 g) Gruyère Cheese, freshly grated
  • 1 tsp Garlic Powder
  • 1/2 tsp Onion Powder
  • 1 tsp Dijon Mustard
  • 1/2 tsp Freshly Ground Black Pepper
  • A small pinch Freshly Grated Nutmeg
  • 3 Tbsp Unsalted Butter, melted (for topping)
  • 1 cup Panko Breadcrumbs
  • 1 tsp Dried Parsley or Thyme (optional)

Instructions:

  1. Preheat the oven to 375°F (190°C). Lightly grease a 9x13 inch (23x33 cm) baking dish with butter.
  2. Slice the potatoes thinly (approx. 1/8 inch or 3 mm). Ensure slices are uniform for even cooking.
  3. Start the Roux: Melt the 4 Tbsp butter in a large saucepan over medium heat. Whisk in the flour continuously for 1–2 minutes until a smooth, pale paste forms, cooking out the raw flour taste.
  4. Build the Béchamel: Slowly pour in the room-temperature milk and chicken stock, whisking constantly to prevent lumps. Bring the sauce to a gentle simmer, stirring until thickened (about 5-8 minutes). It should coat the back of a spoon.
  5. Finish the Cheese Sauce: Remove the pan from the heat. Stir in the Dijon mustard, garlic powder, onion powder, nutmeg, salt, and pepper. Gradually fold in the grated Cheddar and Gruyère, stirring until the cheese is fully melted and the sauce is smooth and glossy. Taste and adjust seasoning.
  6. Layer half of the sliced potatoes in the prepared baking dish in slightly overlapping rows. Lightly sprinkle with salt. Pour half of the prepared cheese sauce evenly over the potato layer.
  7. Repeat the process with the remaining potatoes and salt. Pour the remaining cheese sauce over the top. Gently shake the dish to ensure the sauce settles down between the potato slices.
  8. Initial Bake (Covered): Cover the baking dish tightly with foil. Bake for 35 minutes.
  9. Prepare Topping: While the casserole bakes, combine the melted butter, panko breadcrumbs, and dried herbs in a small bowl.
  10. Uncover and Top: After 35 minutes, remove the casserole. Remove the foil and scatter the panko topping evenly over the surface.
  11. Final Bake (Uncovered): Return to the oven and bake uncovered for another 20–30 minutes, or until the topping is golden brown and bubbling, and the potatoes are fork-tender.
  12. Rest: Let the casserole rest for 10 minutes before serving. This allows the sauce to set up nicely.