Add the chicken pieces to the slow cooker, season with salt and pepper, then cover with 350ml of boiling water or chicken stock. Cook on high for 3 hours, or until the chicken is completely tender. Remove from the water to a plate and shred the meat using two forks, discarding the skin and bones. Set aside.
Add the bacon and onion to the slow cooker. Combine a couple of tablespoons of the water with 1 tablespoon of flour and add this to the slow cooker, stirring it in. Cook on high for 1 hour until the bacon is cooked and the mixture has thickened to a coating consistency.
Return the shredded chicken to the slow cooker and add the parsley and Dijon mustard with some salt and pepper. Mix well and cook on high for 15 minutes to warm through.
Preheat the oven to 200°C. If your slow cooker dish is not ovenproof, decant the mixture into a baking dish. Add the puff pastry to the top of your pie (glazing it with egg, if preferred), then bake the pie for 15–20 minutes, or until the pastry is puffed and golden.
Serve immediately.