Preheat the oven to 180 degrees C. Press one of the pastry shells into and up the sides of a 23cm pie dish.
Bring a saucepan of water to a boil. Add potatoes, and cook until tender, 10 to 15 minutes. Drain, and cut into cubes. Set aside.
Heat 1 tablespoon (20ml) 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 tamari or soy sauce, and bring to a boil. Whisk in rice flour, stirring until there are no lumps. Allow to simmer.
Heat remaining olive oil in a large frypan 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. Cover with the other pastry shell, and crimp edges to seal. Make a few slits in the top pastry to vent steam.
Cook for 40 minutes in the preheated oven, until pastry is golden brown.
You can also use thawed frozen shortcrust pastry sheets for this pie.