Preheat the oven to 180°C/160°C fan/gas mark 4. Heat the vegetable oil in a large pan over a low-medium heat and cook the sliced leeks, stirring regularly until soft for about 10 minutes.
Make up 250ml stock with half a chicken stock cube. Stir through the turkey and crème fraîche then add the chicken stock. Bring to a simmer and season to taste. Simmer for 3 minutes or until slightly thickened.
Place this mixture at the bottom of a shallow 1.5 litre baking dish. Halve the stuffing balls then scatter evenly over. Lay the puff pastry over the top of the dish and cut around it then cut a small hole in the middle to allow steam out.
Brush the pastry with the beaten egg and bake for 20-25 minutes or until the pastry is risen and golden.