Pre - Heat the oven to 200'C
Heat a third of the butter in a large saucepan, cook the leeks for 10mins or until softened, then tip onto a plate and repeat with another third of the butter and the mushrooms.
Whilst the leeks are cooking in a frying pan fry off the Chicken Mince with a drizzle of olive oil, breaking it up with a spatula until cooked all the way through.
Heat the remaining butter in the same saucepan and, once melted, stir in the flour and cook the sandy paste, then stir in the chicken stock a ladleful at a time. Leave on a low heat to simmer for 5 mins.
Then stir the chicken mince, leek, mushrooms, cream and mustard into the sauce, add the parsley and season with salt and cracked black pepper. Spoon into a 20cm pie dish.
Lay the sheet of Puff Pastry on a chopping board, roll out to a desired size and brush with olive oil, then place on top of the pie with the corners resting on the sides of the dish, then sprinkle over salt, thyme leaves and nigella seeds. Press the pastry gently into the edge of the dish.
Bake for about 30mins until the pastry is deep golden, then remove from the oven and serve.
