Melt butter in a medium pan. Stir fry onion for 2 minutes.
Add garlic and stir fry for another 30 seconds.
Add all-purpose flour and continue to stir fry for another 2 minutes.
Now, add mushrooms and parsley. Continue to stir fry for 2 more minutes.
Pour in low fat milk. Cover and bring to a boil.
Season with salt and ground pepper.
Stir for 3 to 4 minutes until sauce is thick and bubbly.