Heat the oil in a large skillet and saute the mushrooms until they begin to wilt.
Add in the garlic and sausage and continue to cook until the sausage is warmed (if pre-cooked, which most are), and the mushrooms are fully wilted. (about 5-10 minutes or so)
When done, turn off the heat, quickly toss in the fresh parsley and transfer to a plate. You don't want to actually cook the parsley. It offers much more flavor if it's quickly tossed in and served.