Preheat a wide, heavy based frying pan. Fry off the diced beef in batches in some oil until it takes on a good amount of colour. Transfer the beef to a bowl.
Add a little more oil to the pan and fry the onions, carrots and garlic.
Add the vegetables and beef into a large, ovenproof dish or pan alongside the beef stock, red wine and rosemary. Transfer to the oven for 2-3 hours until the beef is tender.
To make the pie mix, take a little of the cooking liquor and mix with the cornflour. Return this mixture to the pan of stew and bring to a simmer so the mixture can thicken.
Allow the mixture to cool.
Fry the oyster mushrooms in the butter, season with salt and pepper and stir through the pie mixture alongside the lovage.
Transfer the cooled pie mix to a large pie dish and preheat oven to 180C.
Roll out the pastry until just wider than the dish you’re making the pie in. Brush the underside of the pastry sheet with egg wash to ensure the pastry sticks to the dish. Gently place the pastry on top of the pie dish and press the pastry into the sides of the dish. Egg wash the top of the pastry.
Bake the pie at 180C for 30-40 minutes until the pastry is golden brown and the pie mixture is bubbling. Allow to rest for 10 minutes before digging in.