Before you start, soak the porcini mushrooms in boiling water and set aside.
Peel and finely slice the garlic and onions (keep them separate), and pick the thyme leaves. Dice the potatoes.
Heat a lug of oil in a large pan and fry the garlic, thyme leaves and butter over a medium heat until lightly golden.
Add the onions, turn down the heat and sauté for 10 minutes, or until soft and sticky, but not coloured.
Add the fresh mushrooms, turn up the heat and fry hard and fast for 5 minutes.
Fish out and chop half of the soaked porcini, add to the pan and fry for a few more minutes.
Add 350ml of the stock and the potatoes and season well. Bring to the boil, then turn down the heat and simmer for 30 minutes.
Using a stick blender, blitz the mushrooms just a little – you want them to still be fairly chunky.
Pick and roughly chop the parsley leaves, then stir about half into the mix.
Divide the filling between 6 small pie dishes, reserving 4 heaped tablespoons for later.
Chop the Tunsworth into 1 to 2cm pieces and scatter evenly into each of the pie dishes.
Preheat the oven to 200ºC/400ºF/gas 6.
To make the lids, roll out the pastry on a flour-dusted surface to roughly the thickness of a pound coin.
Cut into 6 pieces, to the shape of your pie dishes, and lay one over the top of each, tucking the edges down slightly. Carefully score each pie lid with a crisscross, then gently press your thumb in the middle of each one to make a well (without puncturing the pastry).
Beat the egg and brush it over the pies, then place in the oven for 15 to 20 minutes, or until puffed up and beautifully golden.
Now to make the sauce. In a pan, stir the mustard and Worcestershire sauce into the remaining filling.
Pour in the leftover porcini liquid, then roughly chop the remaining porcini and add to the mix.
Place the pan over a high heat, pour in the remaining stock and bring everything to the boil. Stir in the rest of the parsley a minute or two before the end, then blitz in a liquidiser until smooth and pour into a jug.
To serve, pour the gravy into the wells of the pies, and serve with a dollop of English mustard.