Dice the lamb into bite size chunks and brown in a tablespoon of olive oil in a pan, or if possible a metal slow cooker with a sear function, alternatively if you have an enamelled cast iron pan that can work on the hob and in the oven then this is a good choice too.
Peel and dice the carrots, and slice the mushrooms, and leek.
To your vessel you'll be using to slow cook (so your slow cooker, or oven dish) add the browned lamb, and vegetables.
Boil the water and pour on top, sprinkle in the stock cube and add the bouquet garni and glass of wine.
Leave to slow cook on a high setting for 4 hours.
When finished, remove the bouquet garni and allow to cool slightly so it has stopped bubbling and add the cornflour mix. If you feel you'd like it thicker, keeping adding a teaspoon of cornflour mixed with a little water at a time until you have your desired thickness. Make sure to heat the sauce back up to simmering if you do this, to make sure the cornflour has thickened. The thickness can vary to personal preference. I like mine a little on the thin side so I can use any leftover as a gravy.
Season with salt and pepper.
Next line your pie dish with a sheet of baking paper on the bottom and over the sides (I prefer individual pie dishes but you can also use a deep frying pan as explained above).
Take your filo sheets and cut into the right size: you want the sheets to be just over twice the width of your pie dish, so when you fold it over the filling they meet at the top and cover it completely.
Baste each filo sheet with olive oil, and layer at different angles until you have around 4 layers, in the pie dish with the sides hanging over the edge.
Take your filling and fill the pie 2/3-3/4 full.
Fold the filo sheets that are hanging over the edge of the dish over the filling, and if you have enough you can twist them together like shown in the picture. Baste the top of the pie with a little more oil.
Bake in the oven at 170'C for 30 minutes or until golden.
For the mash, peel and halve your potatoes and boil for around 25 minutes or until a knife is easily inserted.
Drain thoroughly and mash together with the butter and mustard.
To serve, I like to use any leftover sauce as a gravy for the potatoes and pie.