Put the leeks in a big frying pan with 1 tbsp of the butter and fry gently until really soft and squishy, but not browned. Scrape into a big mixing bowl and leave to cool for 10 minutes.
Meanwhile, melt the rest of the butter in a pan or microwave and preheat the oven to 180°C (160°C fan oven) Gas Mark 6.
Tip the breadcrumbs, chopped parsley and egg into the softened leeks with plenty of seasoning and mix together well. Lay out one of the filo sheets on a clean worktop, brush all over with a little of the melted butter, then carefully stick a second sheet on top. Repeat to sandwich together all the pastry. Use the same brush to now brush the top sheet all over with the grainy mustard. Leave a roughly 3-4cm border all the way around, and spread the leek mixture evenly over the rest. Finally scatter the grated cheese over the top.
Fold the longest pastry borders over the filling to seal in, then roll the strudel up tightly from one of the shortest sides. Carefully lift the strudel onto a baking tray, seam side down, and use the last of the butter to brush all over the top. Sprinkle with the poppy seeds, then bake for 30-35 minutes until golden and crisp. Delicious with a green salad.