Preheat oven to 220℃ / 200℃ Fan / 425℉ / Gas Mark 7
Slice your leek.
Melt the butter in a frying pan over a medium heat and add the sliced leek. Cook with a lid on for 5 minutes until soft but not brown, stirring occasionally.
Stir in the mustard followed by 1 tablespoon of plain flour and cook through for a couple of minutes. (If using chicken stock, you’ll need 2 tablespoons of flour.)
Add the gravy or stock, followed by the milk. Heat through, stirring all the time.
Check for seasoning and add salt and pepper to taste.
Add cooked leftover turkey, cooked ham, carrots and herbs and stir well. Heat through for a few minutes.
Whilst the pie filling is heating through, remove your pastry from the fridge and roll out. Your rolled pastry should be sized to extend beyond all edges of the pie tin by about 3cm (1 ¼ inch). Optional: Use any leftover pastry to cut out star shapes to decorate your pie.
Transfer the pie filling to the pie dish.
Carefully place your pastry on top, cutting away any excess and fluting the pie edges.
Cut a small V shape in the middle of your pastry to allow steam to escape.
Using a pastry brush, brush egg wash onto your pastry shapes and stick them to your pie top. Now glaze the rest of the pastry.
Bake in the preheated oven for 20 to 25 minutes until golden and bubbling.
Serve with peas and mash or homemade crispy oven chips.