1. Preheat oven to 350F and lightly grease a round 10-inch tart pan. Alternatively, you can use a 9-inch glass pie dish if desired.
2. For the crust: Whisk together flax and water mixture in a small bowl and set aside so it can gel up.
3. In a large bowl, stir together the almond meal, oat flour (or buckwheat flour), parsley, oregano, and salt.
4. Add in the flax mixture and oil. Stir until mostly combined, adding the remaining water until the dough is sticky (about the consistency of cookie dough). The dough should stick together when you press it between your fingers.
5. Crumble the dough evenly over the base of the tart pan (or pie dish). Starting from the center of the pan, press the mixture evenly into the pan, working your way outward and up the sides of the pan. Poke a few fork holes in the dough so air can escape.
6. Bake the crust at 350F for 13-16 minutes, or until lightly golden and firm to touch. Set aside to cool while you finish preparing the filling. Increase oven temperature to 375F.
7. For the filling: Place the walnuts in a food processor. Pulse into crumbs, so that no whole walnuts are left.
8. In a skillet, add oil and saute the leek (or onion) and garlic over medium heat for a few minutes. Stir in the mushrooms, season with salt, and cook on medium-high heat until most of the water cooks off the mushrooms, about 10-12 minutes. Stir in the herbs, sun-dried tomatoes, spinach, nutritional yeast, oregano, salt, pepper, and red pepper flakes until combined. Cook until the spinach is wilted. Remove from heat and adjust seasoning to taste if desired.
9. When the vegetables have stopped steaming, add a 1/2 cup of the mixtureto the food processor. Pulse a few times and scrape down the sides. Add the beans and puree until relatively smooth, although the walnuts will still be grainy. Add the cornstarch ( sift first, if very clumpy ) and pulse until thoroughly combined. Transfer the mixture to a bowl and mix in the rest of the vegetables. Spoon mixture into baked crust and smooth out with a spoon until even.
10. Bake for 45 minutes until quiche is firm.. Let cool for about 20 minutes before serving. Serve warm or at room temperature.