Cook the potatoes in boiling water; drain, mash with the milk, season and stir in the mustard
Preheat the oven to 190C/Gas5; grate and reserve 15g of each cheese and cut the rest into cubes; add the cheese and mushrooms to the potato mixture
Brush a sheet of filo pastry with melted butter and sprinkle with one third of the breadcrumbs; repeat the layers, finishing with pastry
Spoon the potato mixture on to the centre of the pastry, leaving a 5cm gap at the edge and at each end; fold these edges over filling and roll up
Place seam side down on a greased baking sheet; brush with remaining butter; sprinkle over the grated cheese and the poppy seeds and sesame seeds; bake in the preheated oven for 40 minutes; serve immediately
4 servings