Mary Berry's potato, leek and cheese pie

Ingredients

• 125g (4½oz) butter
• 125g (4½oz) lard*
• 350g (12oz) plain flour, plus extra for dusting
• 1 egg, beaten, for brushing
• 60g (2½oz) butter
• 2 small leeks (about 150g/5oz each), thinly sliced
• 1 onion, thinly sliced
• 450g (1lb) potatoes (about 2 medium), peeled and cut into 2cm (¾in) cubes
• 600ml (1 pint) milk
• 50g (2oz) plain flour
• 1 tbsp Dijon mustard
• 1 tbsp chopped thyme leaves
• 50g (2oz) mature Cheddar cheese, grated
• 50g (2oz) Parmesan cheese, grated**
• * For a vegetarian pastry you can use all butter, but it won't be as flaky
• ** Or vegetarian alternative Italian hard cheese