Preheat the oven to 200°C.
Prepare the crumble topping by pulsing together the flour, breadcrumbs and butter in a food processor with some seasoning until they resemble fine breadcrumbs. Transfer to a mixing bowl and set to one side.
Prepare the filling by melting the butter in a large saucepan over a medium-high heat. Add the leeks and sweat them with some salt for 4-5 minutes until they start to soften, making sure you stir occasionally. Continue to cook them for another 4-5 minutes, then remove from the heat and stir in the cheese.
Spoon into a large, oval, ceramic baking dish. Sprinkle the crumble topping evenly over and bake for 20-25 minutes until golden brown in colour.
Remove and allow the crumble to sit for a few minutes before serving.
*This dish is very indulgent, so shouldn't be eaten too often. But it's perfect for those special occasions!