Cube and mince the mushrooms with a sharp knife.
In a large skillet, saute the onions in olive oil over medium heat for about 5 minutes.
Add the garlic and mushrooms and cook them for about 10-15 minutes, until all the liquid has evaporated and the mushrooms have started to brown. Boil a large pot of water with salt for the pasta.
At this point, add the tomato sauce, oregano, salt, pepper, and red pepper flakes to the mushrooms. Stir, and cook for 10 minutes. Cook the pasta in the boiling water for about 10 minutes, or until it is al dente.
Reserve some pasta water and use it to thin the bolognese sauce and bring it to a nice consistency (about ½ cup). Add the drained cooked pasta to the mushroom bolognese. Stir everything well, garnish with fresh parsley, and serve.
