Warm the olive oil in a large frying pan. Add the onion and fry for 5 minutes, until soft.
Add the garlic, spinach, chestnut mushrooms, portobello mushrooms, chestnuts, herbs, stock powder, salt, and pepper. Cook for 10 minutes on a low heat. Stir in the vegan red wine, if using. Let cool for 5 minutes.
Place a sheet of the vegan pastry on a chopping board and cut lengthways into 2-centimetre strips. Line a baking dish to the rim with the second sheet, add the filling, then arrange the strips in a cross-hatch pattern over the top. Brush with non-dairy milk.