90g (30z) butter 500g (1lb) leeks, trimmed and thickly sliced 60g (20z) plain flour 300ml (½pint) apple juice 300ml (½pint) milk 1tsp coarse-grain mustard salt and black pepper 4hard-boiled eggs, roughly chopped 150g (50z) mature Cheddar cheese, grated 500g (1lb) potatoes, cut into 5mm (¼in) slices (not peeled) 5 sheets of filo pastry, each about 25 × 38cm (10× 15 in) shallow ovenproof dish, about 25cm (10in) square .
1- Melt 60g (20z) of the butter in a large frying pan, and cook the leeks for about 8-10 minutes until softened. Stir in the flour and cook for 1 minute, then gradually add the apple juice and milk, stirring constantly until boiling. It may look slightly curdled at this stage, but don’t worry, it wll come together. Reduce the heat, and simmer gently for 2-3 minutes. Add the mustard and season well.
2- Remove the sauce the sauce from the heat and stir in the roughly chopped eggs and the cheese. Now cook the potatoes in boiling salted water for4-5 minutes until season, and pour into the dish.
3- Melt the remaining butter. Brush one of the filo sheets with butter and put it over the mixture in the dish, scrunching up the edges to fit. Repeat with the remaining filo sheets, scrunching up the last sheet before putting it on top of the stage, then bake it when you need it.
4- Bake in a preheated over at 200c (400f, Gas 6) for 30-40 minutes until the filo is crisp and golden and the pie is hot right through.