Preheat the oven to 200°C/Gas 6.
Melt the butter in a frying pan, add the olive oil to stop it burning and fry the onions until slightly browned, about 5 minutes.
Add the mushrooms, bay leaves and rosemary and cook for a further 4-5 minutes.
Add the chestnuts, red wine and vegetable stock. Bring to the boil and simmer for 20-30 minutes until the onions are soft.
Add the beurre manie, stirring constantly until is dissolves, and cook for a further 5 minutes until the sauce is thickened.
Stir in the mustard and flat leaf parsley and season to taste with salt and freshly ground black pepper. Spoon the mixture into a pie dish.
Roll out the pastry on a floured surface and place on top of filling, crimping the edges to seal the pie. Cut a cross in the top of the pastry to let the steam out during cooking.
Bake for about 20 minutes until golden. Serve immediately.