Prepare the Crust: In a large bowl, combine the mashed potatoes, oat flour, tapioca starch, ground flaxseeds, salt, and baking powder (if using). Add the unsweetened applesauce and mix until a dough forms. If the dough is too dry, add 1 tablespoon of cold water until the dough is smooth but firm enough to work with. Roll the dough between two pieces of parchment paper** to your desired thickness, or press the dough evenly into your pie dish to form the bottom crust***. Reserve some dough for the top if making a double crust.
Prepare the Soy Curls: Soak the soy curls in 1 ½ cups of warm vegetable broth mixed with 1 tablespoon of vegan chicken bouillon powder for 10 minutes. Drain the soy curls, squeeze out any excess liquid, and reserve some soaking broth to dissolve the cornstarch. Set aside.
Prepare the Filling: In a large pan over medium heat, sauté the diced onion, garlic, carrots, potatoes, and celery with a splash of vegetable broth. Cook until the vegetables are soft, about 8-10 minutes. Add the soy curls, peas, thyme, rosemary, bay leaf, salt, and pepper. Stir well to combine. Add the remaining 1 tablespoon of vegan chicken bouillon powder to the filling and mix well. In a small bowl, dissolve the cornstarch in the reserved broth from soaking the soy curls. Add this mixture to the pan and cook until thickened about 5-7 minutes. Adjust seasoning as needed. Remove the bay leaf before assembling the pot pie.
Assemble the Pot Pie: Pour the prepared filling into the uncooked crust. If using a double crust, roll out the remaining dough and lay it over the filling. Seal the edges and make a few small slits on top to allow steam to escape. Optional: Brush the top of the crust with the plant-based milk and maple syrup mixture to help it brown in the oven.
Bake the Pot Pie: Bake the pot pie in the oven at 375°F (190°C) for 30-35 minutes or until the top crust is golden and firm. Let it cool for 5-10 minutes before serving.
Garnish and Serve: Garnish with fresh parsley if desired.