Preheat oven to 350 degrees. Press one of the pie crusts into and up the sides of a 9 inch pie plate.
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 tbsp 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 a boil. Whisk in rice flour, stirring until there are no lumps. Allow to simmer.
Heat remaining olive oil in a large skillet over medium-high heat. Add Portobello pieces, and sauté 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 crust. Cover with the other pie crust, and crimp edges to seal. Make a few slits in the top crust to vent steam.
Bake for 40 minutes or until crust is golden brown.