Cook pasta in very salty water according to recipe or package directions.In a large saucepan, heat olive oil over medium heat.Add mushrooms, garlic, green onions, salt and pepper to oil and saute until mushrooms have reduced in size, about 4 minutes. If the mushrooms soak up all the olive oil, add another tablespoon.Add flour to saucepan and stir to incorporate all ingredients. Cook for one minute.Add ½ cup of the milk and stir to break up any lumps of the mushroom and flour mixture. The milk will thicken fairly quickly; that’s okay. Add the remainder of the milk and stir to incorporate all ingredients. Bring to a simmer and cook until milk has thickened, about 5 minutes.Remove sauce from heat and stir in parmesan cheese. If you can, shred your own cheese – pre-shredded cheese has additives in it that make it more difficult for the cheese to melt smoothly. Stir in the chopped parsley.Garnish with extra parsley and shredded parmesan cheese and serve immediately.