Using a food processor, pulse the mushrooms until they are very finely chopped. In a large saucepan add a drizzle of olive oil and the mushrooms.Sauté until the mushrooms give off all their water and begin to brown. Remove from the heat and set aside in a bowl. In the same saucepan sauté the onion in a little olive oil. Cook until soft. Add the celery, carrot, zucchini, garlic and dried oregano. Cook until the vegetables are very soft. Use your spoon or spatula to mash them in the pan so they break down. Add the tomato paste and cook stirring constantly for 2-3 min. Add the tinned tomatoes and stock, bring to a gentle simmer. Allow to simmer for 30 min until reduced and thickened. Taste for seasoning. In a large pot of boiling salted water, boil the spaghetti according to package instructions. Once al dente, drain the pasta. Divide the spaghetti between bowls and top with the mushroom bolognese. Sprinkle bowls with Parmesan cheese and fresh parsley.