Add the pancetta to a large pan with the heat off. Turn to medium and gently fry until crispy, with the fat rendered out. Remove and place to one side, leaving the excess fat behind.
Add the diced leek and sweat it right down until soft and starting to turn golden. Melt the butter, then stir in the flour. Add the milk, 1/2 a cup at a time, whisking as you go to make sure no lumps form. Stir in the Boursin until it melts, then stir in the mustard.
Stir in the chicken/turkey and pancetta, then simmer for around 7-8 minutes until everything is nice and thick. It will be pretty thick at the start, but then it loosens up, so you will need to simmer to reduce it and thicken it back up again. Check for seasoning and adjust if needed, then leave to completely cool to room temp.
Whilst the filling is cooling, preheat the oven to 200C/400F.
Roll out the pastry and slice the whole sheet into 4 rectangles, then slice the half sheet into 2 more rectangles (6 in total). Add the filling to one side of each rectangle, then fold over the other side to secure the filling (see video below for reference). Use a fork to crimp around the edges, then clean up with a knife if needed.
Place on a large tray lined with baking/parchment paper (you can use the paper with the pastry), then brush with egg wash. Slice a few steam holes, then bake in the oven for around 20 minutes, or until deep golden and visibly crisp.
Tuck in and enjoy (careful, the filling will be very hot).