Preheat the oven to 180 C / Gas mark 4. Press one of the pastry sheets into and up the sides of a 23cm pie dish.
In a saucepan, add the potatoes and water to cover. Bring to the boil and cook until tender, 10 to 15 minutes. Drain, and cut into cubes. Set aside.
Heat 1 tablespoon of olive oil in a large saucepan over low heat. Add onion and shiitake mushrooms, cover, and let the mushrooms sweat for about 7 minutes, stirring occasionally. Pour in the water and soy sauce, and bring to the boil. Whisk in rice flour, stirring until there are no lumps. Allow to simmer.
Heat remaining olive oil in a large pan over medium-high heat. Add portobello pieces, and saute briefly until mushrooms are browned on the outside. Add mushrooms to the gravy mixture along with the carrots, celery and potatoes. Simmer for about 10 minutes, stirring occasionally. Season with thyme and sage. Pour the mixture into the prepared pastry case. Cover with a sheet of pastry, and crimp edges to seal, cutting off any excess. Make a few slits in the top pastry to vent steam.
Bake for 40 minutes in the preheated oven, until pastry is golden brown.
