Ingredients:

  • 5 kg (3.3 lbs) Yukon Gold potatoes, peeled
  • 450 g (1 lb) Asparagus spears, trimmed and halved
  • 60 g (1/2 cup) Frozen peas
  • 1 Tbsp (15 ml) Olive oil
  • 1/2 tsp (2.5 g) Kosher salt, plus more for seasoning
  • 60 g (4 Tbsp) Unsalted butter
  • 60 g (1/2 cup) All-purpose flour
  • 950 ml (4 cups) Whole milk, warmed slightly
  • 1/4 tsp (1 g) Freshly grated nutmeg
  • 5 tsp (7.5 g) Kosher salt
  • 1/2 tsp (2.5 g) Freshly ground black pepper
  • 2 cloves Garlic, finely minced
  • 1 Tbsp (5 g) Fresh thyme leaves
  • 200 g (7 oz) Gruyère cheese, grated
  • 100 g (3.5 oz) Mature Cheddar cheese, grated (reserve 50g for topping)
  • 1 tsp (5 ml) Lemon zest, finely grated

Instructions:

  1. Preheat oven to 190°C (375°F). Lightly grease the 9x13 inch baking dish. Slice the peeled Yukon Gold potatoes into uniform, very thin discs (2mm thickness is ideal).
  2. Quickly blanch the asparagus in boiling, salted water for 60 seconds. Immediately plunge into an ice bath to stop cooking and drain well. Gently toss the sliced potatoes, blanched asparagus, and frozen peas with 1/2 tsp of salt and a drizzle of olive oil in a large bowl.
  3. Make the Roux: In a large saucepan, melt the butter over medium heat. Whisk in the flour until smooth. Cook for 1–2 minutes, stirring constantly, until it smells slightly nutty.
  4. Add Milk: Gradually whisk the warmed milk into the roux, a bit at a time, ensuring the mixture is smooth before adding more. Bring to a simmer, whisking constantly, until the sauce thickens enough to coat the back of a spoon (about 5–7 minutes).
  5. Melt Cheese: Remove the pan from the heat. Stir in the garlic, thyme, nutmeg, 1.5 tsp salt, and pepper. Add the grated Gruyère and 50g of the Cheddar (reserving the rest). Stir until the cheese is completely melted and the sauce is smooth and glossy. Stir in the lemon zest last.
  6. Combine and Layer: Pour approximately one-third of the cheese sauce into the bowl with the potatoes and vegetables and gently toss everything to coat. Transfer half of the coated potato mixture to the prepared baking dish, spreading evenly. Pour half of the remaining sauce over the layer.
  7. Finish Layering: Add the remaining potato mixture, pressing down gently. Pour the last of the cheese sauce over the top, making sure it seeps down between the slices.
  8. Bake: Cover the dish tightly with foil. Bake for 30 minutes. Remove the foil, sprinkle the remaining 50g of Cheddar cheese over the top. Return to the oven and bake for another 20–25 minutes, or until golden brown and bubbly, and the potatoes are tender.
  9. Rest: Remove the gratin from the oven and let it rest for 10 minutes before slicing and serving.