Peel the potatoes, cut into chunks and cook in a large pan of boiling water until tender; drain, leave for a few minutes then coarsely mash; stir in a third of the butter and some seasoning; transfer to a bowl and leave to cool
Thinly slice the leeks and peel and finely chop the garlic; melt the remaining butter in a large frying pan over a medium-low heat, add the leeks, garlic and thyme, season and fry for about 10 minutes until softened, stirring occasionally; transfer to a bowl and leave to cool
Preheat the oven to 200C/Gas6; roll out 2/3rd of the pastry into a circle big enough to line the 20cm cake tin with about 5cm overhang all round. Line the tin with the pastry
Spoon in the mashed potato and gently press down, crumble the cheese on top and then spoon in the leeks
Roll out the remaining pastry and using the tin as a guide, cut out a pie lid to it on top of the leeks; tuck this in place and brush the top with the egg mixture. Now trim the pastry sides to give a 3cm rim all round; fold this down over the lid to seal the pie, then brush the rim with egg; decorate if you wish with the pastry trimmings
Bake for 40-45 minutes until risen and golden; serve hot or warm
Servings 0