how to make mushroom sauce: our basic order of operations.
- saute the mushrooms. ...
- add onion & garlic. ...
- make a roux with a bit of flour to help thicken the sauce.
- add milk, stir to work out any lumps of flour, and cook until thickened.
- stir in some parmesan, pour your mushroom sauce over pasta/chicken/etc., and enjoy!