Heat olive oil over medium heat. Add mushrooms to oil and sauté until mushrooms have reduced in size, about 4 minutes. Add garlic, onions, salt and pepper to mushrooms and stir to combine. Cook about one minute.
Add flour to the vegetables and stir to incorporate all ingredients. Let cook for 30 seconds.
Add 1/2 cup of the milk and stir to break up any lumps of the mushroom and flour mixture. As soon as the milk thickens, add the remainder of the milk and stir to incorporate all ingredients. Simmer for 5 minutes.
Remove sauce from heat and stir in parmesan cheese. Stir in the chopped parsley.
Cook Perfetto pasta according to the instructions on the box.
Toss the pasta with the mushroom sauce. Garnish with extra parsley and serve immediately.