In a small bowl whisk together 2 cups water with the cornmeal, quinoa and salt. Bring the remaining 2 cups water to a boil in a heavy saucepan. Stir the cornmeal mixture into the boiling water, and continue stirring. Be careful! The polenta may spit and sputter and the hot bits of polenta can burn. Turn the heat to very low and cook the polenta for 40 minutes; stirring with a wooden spoon every 10 minutes.
Make the tomato sauce while the polenta cooks: Heat olive oil in a large saucepan over medium heat, add the onions. Cook the onions for about 5 minutes, stirring often, until they have softened. Stir in the chopped mushrooms and the garlic, saute for 5 minutes. Then add the tomatoes, sherry, chili flakes and oregano. When the sauce comes to a boil turn the heat to low. Simmer the sauce for 30 minutes; season with salt and pepper.
When the polenta has cooked for 40 minutes, sprinkle the green onions and grated cheese into the polenta. Stir well. Mound the polenta on plates, make a well in the center of each mound and ladle in the tomato sauce. Garnish with the shaved cheese.
