If using skin-on thighs: Preheat oven to 180C/350F. Use your hands to remove the skin and lay it flat on a large baking tray. Sprinkle with a pinch of salt, then place baking paper on top. Place another baking tray on top (this will prevent the skin from curling up). Bake in the oven for 20-25mins or until golden and crisp. Remove and pat with kitchen roll to soak excess fat and finish with another pinch of salt. The skin will crisp as it cools. Meanwhile, carefully cut out of the bone from the thighs.
Dice the chicken and season all over with salt and pepper.
Add the bacon to a large pan over medium heat and fry until crisp with the fat rendered. Remove and place in the baking dish, leaving the fat behind. Raise heat slightly to medium-high.
Space the chicken out in the pan and fry until golden on each side. Don't worry too much about cooking it right through, just a few minutes each side to lightly brown. Remove it from the pan and place in the baking dish with the bacon, leaving the excess fat behind.
Lower heat to medium, then add the onion and fry for a couple of minutes until it begins to soften. Add the mushrooms and continue frying until the onions are soft & golden and the mushrooms have built up a golden crust. Add the garlic and fry for 30-60 seconds more.
Melt in the butter then stir in the flour to form a roux. Gradually pour in the stock and cream, stirring as you go to prevent lumps forming. Stir in the mustard, thyme, chicken and bacon and bring to a gentle simmer. Reduce heat slightly and simmer for 10 minutes to thicken the sauce, stirring occasionally.
Once thickened, adjust seasoning if desired then pour everything into the baking dish and leave to cool for 15 minutes. Meanwhile, preheat the oven to 200C/400F.
Once the filling has cooled, brush around the rim of the dish with egg wash. Place the puff pastry over the top and crimp around the outside to 'glue' it to the dish. Trim excess pastry, then brush with egg wash. Slice a few steam holes, then place in the oven for 20mins, or until deep golden and crisp. Leave for 5 or so mins to cool, then serve up with the crispy chicken skin and enjoy!