Preheat the oven to 200 °C (180 °fan/ Gas 6).
Prepare your leeks by discarding the thick outer leaves, and removing any dirt by rinsing them under cold running water. Top and tail the leeks to remove the root and also the dark green top half. All you want left are the white and pale green parts of the leek. Chop the leeks into 5 cm chunks.
4 Leeks
Bring a pan of salted water to the boil, add the leeks to the water, return to the boil for 5 minutes. Drain and set aside.
Make the cheese sauce (using the roux method). Melt the butter in a pan, stir in the flour, then whisk in the milk gradually. Keep whisking until the sauce starts to thicken and it coats the back of a spoon. Remove the sauce from the heat and stir in the grated cheese, nutmeg, and dijon mustard. Season with salt and pepper.
30 g Butter, 30 g Plain Flour, 350 ml Milk, 100 g Extra Mature Cheddar, 1 teaspoon Dijon Mustard, ⅛ teaspoon Nutmeg
Assemble the leeks in a baking dish. Spoon a small amount of the cheese sauce in the bottom of the baking dish (2 tablespoons should be enough), lay the leeks into the dish, then pour over the rest of the cheese sauce. Sprinkle over the breadcrumbs, Parmesan, and salt & pepper, and bake in the oven for 20-25 minutes until golden and bubbling.
2 tablespoon Breadcrumbs, 25 g Parmesan, Salt and Pepper