Set oven to 180C. Mix the salt and flour in a large bowl, in a separate jug, combine the olive oil and warm water. Gradually add the liquid mixture to the dry one, combining the ingredients as you go until you have a soft dough, taking care not to overmix. Cover the dough with a tea towel and let it sit for 20 mins.
Whilst the dough rests prepare the filling. Heat the oil in a frying pan on medium heat. Once hot add in the onions, mushrooms and garlic and cook until they are softened, about 15 minutes. Season the vegetables with a pinch of chilli flakes and salt to taste. Add the spinach into the mixture and cook until wilted. Crumble in the tofu, breaking it up with the spoon as you go. Mix in the tomato and olive tapenade, the preserved lemon and pine nuts and then set aside.
To form the pie crust put the dough onto a clean worksurface, dusted with flour. Roll the pastry out into a circle of 5mm thickness. Put the pie filling into the dish and place the casing over the top. Crimp the edge of the pastry and brush the top of the pie with some vegan milk and sprinkle on the black onion seeds.
Pierce the top of the pie to create a hole for steam to come out – a skewer works best for this. Place in the oven for one hour, until the pastry is golden and crisp.