Ingredients:
- 4 medium (approx. 900g) Leeks
- 2 tablespoons Unsalted Butter (for leeks)
- 1 teaspoon Fresh Thyme leaves
- 1/4 cup Dry White Wine
- Salt and Freshly Ground Black Pepper, to taste
- 3 tablespoons Unsalted Butter (for sauce)
- 3 tablespoons All-Purpose Flour
- 2 cups Whole Milk (warmed slightly)
- 1 cup Gruyère Cheese (freshly grated)
- 1/4 cup Parmesan Cheese (finely grated)
- 1 teaspoon Dijon Mustard
- 1/8 teaspoon Freshly Grated Nutmeg
- Salt and White Pepper, to taste
- 1/2 cup Panko Breadcrumbs
- 1 tablespoon Melted Unsalted Butter (for topping)
Instructions:
- Preheat oven to 375°F (190°C). Thoroughly clean and slice leeks into uniform ½-inch rounds.
- Melt 2 tbsp butter in a large skillet over medium heat. Add leeks and thyme. Sauté gently until soft and translucent, about 8–10 minutes (do not brown).
- Pour in the white wine, scrape up any browned bits, and cook until the wine has almost completely evaporated. Season lightly with salt and pepper. Set aside.
- Make the Roux: In a medium saucepan, melt the 3 tbsp butter over medium heat. Whisk in the flour continuously for 2 minutes until a smooth paste forms.
- Build the Béchamel: Slowly whisk in the warm milk, a splash at a time, ensuring no lumps form. Bring to a gentle simmer, whisking constantly, until the sauce thickens enough to coat the back of a spoon (about 5-7 minutes).
- Finish the Sauce: Remove from heat. Stir in the Gruyère, Parmesan, Dijon, nutmeg, salt, and pepper until smooth. Taste and adjust seasoning.
- Assemble the Gratin: Gently fold the cooked leeks into the cheese sauce until evenly coated. Transfer the mixture to a lightly buttered 8x8 inch gratin dish.
- Prepare the Topping: In a small bowl, toss the Panko breadcrumbs with 1 tbsp melted butter. Sprinkle evenly over the top of the leeks.
- Bake for 35–40 minutes, or until the sauce is bubbling vigorously and the topping is deep golden brown and crisp.
- Let the gratin rest for 10 minutes before serving to allow the sauce to set slightly.