Add the butter to an oiled pan to unsure it doesn’t burn. Once warm, add your sliced leeks to the pan and cook for 5 minutes on a medium low heat with some salt and pepper to soften.

Next introduce the flour and stir until it thickens. Then slowly add the milk, stirring continuously until combined. Cook for a few minutes until the sauce has thickened enough to coat the back of a spoon. Stir in around three quarters of the mature cheddar, the mustard and mix to combine. Taste, and add salt and pepper if needed

Tip everything into a baking dish. Sprinkle the cheese and breadcrumbs over the top and bake for 20 minutes or until golden and crisp on top.