1. Preheat oven to 190°C. Place the butter in to a large sauté pan and turn the heat to medium. When melted, add the sliced leeks, stir around in the butter and cook for 10-12 minutes, stirring all occasionally.
2. When the leeks are soft, add the flour and stir around in the leeks for a few minutes. Now add half of the milk, stir well and bring to the boil. When the milk starts to thicken, add the remaining milk and repeat the process. Add the grated cheese, the mustard, the chopped parsley, season with salt and pepper and leave to cool.
3. Make the crumble topping by combining the flour with the oats before adding the diced butter and combining together using your fingers. Once the mix resembles bread crumbs, add the remaining ingredients and mix well.
4. Pour your leeks into a baking dish and top with the crumble. Place into a hot oven to cook for 30 minutes until golden brown and bubbling. Serve straight from the oven to the table.
Our thanks to www.britishleeks.co.uk for permission to use this recipe.