Preheat oven to 180C
In a large fry pan, melt butter and cook the leek until softened. Sprinkle over the flour, stir to combine, then add the milk and stir until smooth. Add the mustard and season with salt and pepper. Remove from heat, then fold through the Huon Salmon.
To make the topping, in a large bowl, add the flour then rub in the butter until the size of peas. Stir in the cheese, parsley and season with salt and pepper.
Place the salmon filling in a 1 litre capacity baking tray. Top with crumble mix and bake in the oven for 30 minutes or until the crumble is golden and the edges are starting to bubble.