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Preheat the oven to 220°C/Fan 200°C/Gas 7. Lightly butter a shallow baking dish. (It will need to hold around 1 litre.)
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A third-fill a medium saucepan with water and bring to the boil. Remove any tough or damaged leaves from the outside of the leeks if not already done. Trim the leeks, leaving all of the white part and some of the green.
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Cut the leeks into roughly 3cm pieces and add to the water. Return to the boil and cook for 5 minutes, depending on the thickness, or until the leeks are tender. They are ready when the tip of a knife slides in easily. Drain in a colander over a large mixing bowl to reserve the cooking water.
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To make the sauce, return the empty pan to the heat, add the butter and melt gently. Stir in the flour and cook for 10–15 seconds stirring. Gradually add the milk and then a ladleful (around 100ml) of the reserved cooking liquid, stirring or whisking well between each addition to make a smooth, creamy sauce.
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Add the mustard, half the cheese and season generously with salt and pepper. Simmer gently for 1–2 minutes, stirring constantly.
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Scatter the leeks into the baking dish and pour the sauce on top. Mix the remaining cheese with the breadcrumbs, if using, and sprinkle on top. Bake for 12–15 minutes or until bubbling and golden brown.
