First of all, you need to trim the turkey meat into even-sized pieces – roughly ½ inch (1 cm) squares.
Then melt the butter in a medium-sized saucepan and fry the chopped onion over a medium heat until it has turned pale gold – about 4 minutes. Then, using a draining spoon, remove it to a plate. Now turn the heat up to high and add the strips of bacon and fry these, tossing them around a few times, for about 4 minutes until they are really crisp.
After that, transfer the bacon to join the onion on a plate and start to fry the turkey meat in the bacon fat. You'll need to do this in two batches, and each batch should take about 2 minutes to turn golden on all sides. Now return the onion and bacon and the first batch of turkey to the saucepan. Then, using a wooden spoon, and keeping the heat at medium, sprinkle in the flour and stir it in to soak up all the juices.
After that, add the chicken stock a little at a time, stirring as you add until all the stock has been incorporated. Then, as it comes to simmer, it will have thickened to a creamy sauce. At this stage add the mushrooms, season well and then let it simmer at the lowest possible heat, uncovered, for 30 minutes, giving it a stir from time to time. Then, finally, stir in the crème fraîche and parsley, taste to check the seasoning and then leave it aside until it is completely cooled (preferably chilled).
All of this can be done well ahead of time, but when you want to make the feuilletés, pre-heat the oven to gas mark 6, 400°F (200°C). Allow the pastry to come to room temperature – this will take about 10 minutes – then carefully unroll the sheet of pastry and cut in half so that you have two pieces. Roll one of these out until it measures 18 x 16 inches (45 x 40 cm) and then trim it to give two 7 x 7 inch (18 x 18 cm) squares.
Gather up the trimmings and reserve these, then repeat the whole process with the other piece of pastry so that you have four squares in all. Now divide the cold filling between the four pieces of pastry, placing it in the centre. Next, brush the edges with the beaten egg yolk, pull up opposite corners to meet in the centre so that what you have is in fact an envelope. Then pinch the seams together carefully, as you don't want them to burst open. Now make a small hole in the centre to allow the steam to escape, then re-roll the pastry trimmings quite thinly and cut them into leaf shapes, making veins in the leaves with the back of a knife. Then arrange the leaves to decorate the parcels.
You can make the feuilletés up to this stage well in advance and chill them. Before cooking, brush the whole lot thickly with a beaten egg yolk. They will need to go on to a greased baking sheet and bake on a high shelf in the oven for 20-25 minutes or until they are a rich golden brown. All this needs is a lightly cooked green vegetable or a salad.
You will also need a lightly greased baking sheet, 10 x 14 inches (25.5 x 35 cm), and a tape measure.