Preheat the oven to 350°F. Lightly oil a 9-inch round dish with minimum 2-inch high sides.
Make the almond parmesan. In a food processor, combine the ½ cup sliced almonds, nutritional yeast, salt, garlic powder, and lemon zest. Pulse until you have a fine meal. Transfer to a bowl and set aside.
Make the almond ricotta. First, drain the almonds. Then, in the food processor (no need to rinse), combine the drained almonds, hot water, lemon juice, olive oil, apple cider vinegar, garlic, and salt. Run the motor until you have a smooth, ricotta-like texture, scraping the sides down a couple times. Set aside.
Make the polenta crust. In a medium saucepan, combine the water, vegan butter, red pepper flakes, and salt. Bring the mixture to a boil over medium heat. Grab a whisk and slowly start pouring the cornmeal into the boiling water in a steady stream, whisking the whole time. Once you’ve poured out all the cornmeal and there are no visible clumps, switch to a rubber spatula. Bring the polenta to a boil and then lower to a simmer. Cook, stirring very often, until quite thick but still slightly fluid, about 15 minutes. Stir in ¼ cup almond parmesan and adjust seasoning.
Remove the polenta from the heat and scrape it into your prepared round dish. Working quickly with the spatula, push the soft polenta up the sides of the dish to form a “crust.” Make it as even as you can and smooth out the bottom completely. The consistency should be like Play Doh and quite easy to work with. Set aside once complete.
Make the mushroom filling. Heat a large skillet over medium heat. Add the 1 tablespoon of olive oil. Add the onions to the skillet and cook, stirring often, until quite soft and translucent, about 4-5 minutes. Add the mushrooms to the skillet and let them sit for a full minute. Stir the mushrooms up and let them sit for another full minute. Keep stirring the mushrooms until they’re soft and glistening. Then, add the garlic and stir until fragrant, about 30 seconds. Add the spinach and cook until just-wilted, about 1 minute. Season the mushroom mixture with salt and pepper and stir. Remove from the heat.
Assemble the spinach and mushroom polenta pie. Spread half of the almond ricotta across the bottom of the polenta crust. Using a slotted spoon, transfer the mushroom and spinach mixture to the pie on top of the ricotta, leaving as much moisture in the skillet as you can. Then, spoon the marinara sauce on top of the spinach and mushrooms. Dollop the remaining almond ricotta on top. Sprinkle a bit of almond parmesan and brush a bit of olive oil on the exposed polenta crust if you like.
Bake the spinach and mushroom polenta pie for 25 minutes, or until sufficiently warmed through. Turn the oven to broil and broil the pie for an additional 3 minutes to brown the top.
Serve the spinach and mushroom polenta pie hot with chopped basil on top and extra almond parmesan.