👨‍🍳 Instructions

1. 1

   Preheat oven to 375°F (190°C). Lightly butter a 9x13 inch baking dish.

   💡 Tip: Ensuring the dish is buttered prevents sticking.
2. 2

   In a large saucepan, melt the butter over medium heat. Add the sliced leeks and cook until softened, about 5-7 minutes, stirring occasionally.

   ⏱️ 7 minutes

   💡 Tip: Washing the leeks thoroughly is important to remove any grit.
3. 3

   Sprinkle the flour over the leeks and cook, stirring constantly, for 1 minute to form a roux.

   ⏱️ 1 minute

   💡 Tip: Cooking the flour for a minute removes the raw flour taste.
4. 4

   Gradually whisk in the warm milk until the sauce is smooth and thickened. Bring to a gentle simmer and cook for 2-3 minutes, stirring.

   ⏱️ 3 minutes

   💡 Tip: Warming the milk helps to prevent lumps.
5. 5

   Remove from heat. Stir in the chopped dill, Dijon mustard, salt, and white pepper. Taste and adjust seasoning if necessary.

   💡 Tip: Adding dill off the heat preserves its fresh flavor.
6. 6

   Arrange half of the sliced potatoes in an even layer in the prepared baking dish. Top with half of the flaked smoked fish and half of the leek sauce.

   💡 Tip: Overlapping the potato slices slightly ensures even coverage.
7. 7

   Repeat the layers with the remaining potatoes, smoked fish, and sauce.

   💡 Tip: Ensure the top layer of potatoes is well covered with sauce.
8. 8

   If using, sprinkle the grated Gruyère cheese evenly over the top.

   💡 Tip: A cheese topping adds a lovely golden crust.
9. 9

   Cover the dish loosely with aluminum foil. Bake for 30 minutes.

   ⏱️ 30 minutes
10. 10

    Remove the foil and bake for another 20-25 minutes, or until the potatoes are tender and the top is golden brown and bubbly.

    ⏱️ 25 minutes

    💡 Tip: Baking uncovered allows the top to brown and crisp up.
11. 11

    Let the gratin rest for 5-10 minutes before serving. Garnish with fresh chopped parsley.

    ⏱️ 10 minutes

    💡 Tip: Resting allows the gratin to set, making it easier to serve.