In a small bowl, whisk together 1/2 the water with the polenta, quinoa and salt. Bring the remaining water to the boil in a heavy saucepan. Stir the polenta mixture into the boiling water and continue stirring. Be careful! The polenta may spit and sputter and can burn. Turn the heat to low and cook the polenta for 40 minutes; stirring with a wooden spoon every 10 minutes.
Make the sauce while the polenta cooks: Heat olive oil in a large saucepan over medium heat, add the onions and cook for about 5 minutes, stirring often, until they have softened. Stir in the mushroom and garlic and cook for 5 minutes. Then add the tomatoes, sherry, chillies and oregano. When the sauce comes to the boil, turn the heat to low and simmer for 30 minutes. Season with salt and pepper.
When the polenta has cooked for 40 minutes, stir in the spring onions and grated cheese. Mound the polenta on plates, make a well in the centre of each mound and ladle in the sauce. Garnish with the shaved cheese.