In a pot, boil water, add pasta, and cook for 7-8 minutes till al dante. Strain and reserve some pasta water.
In the same pot, melt butter, add onion and garlic, and cook till translucent. Then add mushrooms and cook till their moisture has evaporated.
Add all-purpose flour and mix well. Pour in vegetable stock, salt, and pepper and simmer it for 10 minutes.
Cool it down and blend the mixture into a smooth soup. Again pour the soup into the pot, and add milk and heavy cream. Mix well.
Add the boiled pasta to the soup, and mix well. Adjust the consistency with pasta water, and adjust the seasonings.
Your mushroom pasta soup is ready to be served.