Place the soaked chestnuts, herbs and 150ml/5fl oz of wine in a saucepan with vegetable stock to cover and cook until just tender - approximately 50-60 minutes. Drain the chestnuts, reserving the liquid.
Melt the butter in a frying pan and sauté the onions until slightly browned.
Add the mushrooms and cook for a further 4-5 minutes.
Add the chestnuts, the remaining red wine and sufficient chestnut cooking liquor to cover.
Bring to the boil and simmer for 20-30 minutes to reduce the liquid a little.
Stir in the mustard, tamari and black pepper to taste. Cook for a further 5 minutes.
Check seasoning and adjust as necessary.
Spoon the mixture into a pie dish. Roll out the pastry on a floured surface and place on top of filling.