Add the minced beef to a preheated saucepan and fry quickly releasing the fat (if you use lean beef you'll have to use oil) add thinly sliced onion and lower the heat allowing onions to saute - you can use more meat if you want, but I buy 450g and steal 100g of it to make my Cornish pasties ;)

Sprinkle the flour over the top of the neat and onion mixture and stir in to absorb the butter. Cook for 1 minute whilst stirring.

Add the beef stock a little at a time stirring well in between to avoid any lumps.

Now make the leek layer (leaving meat to cool helps keep the layers separate during cooking)

When the leeks are soft, sprinkle over the flour and stir in for 1 minute

If you don't have mustard powder you can use yellow English mustard. Add any seasoning at this point that you feel necessary - we all have different palates

So mashed potato - i may make fresh for this meal, but often I use leftover, so up to you but I would say that you need it to be fairly stiff.

You can keep in the fridge up to 48 hours before baking, the longer the better as it firms up the layers