Add a drizzle of oil to a large pan over medium-high heat. Space out the leeks & fry until lightly charred on both sides then remove. Top pan up with oil if needed.

Season chicken with ½ tsp salt & pepper then space out in the pan. Fry for a few mins each side to build up a golden crust then remove & place to one, leaving the excess fat behind.

Add mushrooms & fry until they build up golden crust then season with a good pinch of salt & pepper. Lower the heat to medium then add the butter & garlic and stir to melt the butter and lightly fry the garlic (30 secs or so). Stir in the flour to form a roux then gradually pour in the stock and cream, stirring as you go to prevent lumps forming.

Stir in the leeks & chicken (alongside the resting juices) then stir in the mustard, thyme & nutmeg. Bring to a simmer then reduce the heat very slightly and stir occasionally until the sauce is nice & thick (12 or so mins). Adjust seasoning if needed then pour the filling into a suitably sized baking or pie dish & leave to cool for 15 mins so a 'skin' forms on top. Meanwhile, preheat the oven to 200C/400F.

Place the pastry on top & slice off excess. Use a fork to crimp around the outside then brush with egg wash. Slice a few steam holes then place on a baking tray and bake for around 20mins, or until deep golden and crisp. Leave for 5 or so mins to cool then tuck in & enjoy!