Soak the dried mushrooms in hot water for 30 minutes.
Heat the oven to 200C/400F/Gas 6.
Shred the meat from the chicken and finely chop.
Finely slice the shallots or onion. Drain the dried mushrooms, trim off the stalks and finely slice. Trim and finely slice the fresh mushrooms if using.
Heat the oil, and cook the shallots gently until soft. Sprinkle with the flour, and cook, stirring constantly, for 2 minutes, then gradually add the chicken stock, stirring.
Add the mushrooms and soy sauce, oyster and hoi sin sauces and simmer for 5 minutes. Remove from the heat, stir in the chicken and coriander leaves, and leave to cool.
Thaw the pastry, roll out, and cut into 8x15 cm squares. Plop 1 hearty tbsp of the mixture on four of the squares, top with the remaining squares, and press down around the filling. Place a large glass or small Chinese bowl over the top, and trim into a circle.
Brush with egg yolk, cut a small slash in the top, and bake for 20 minutes or until golden.
Serve with julienned mange tout on the side.
