Rub mushrooms lightly with a paper towel to clean; then slice and set aside.
Heat oil in large saute pan for about 5 minutes, until just before oil starts to smoke; then add the butter.
After the butter has melted, add all of the mushrooms, garlic, salt and pepper to taste; add another tablespoon of butter or oil, if desired. Saute on high heat for about 5 minutes. Add in lemon juice and mix; be careful not to overcook the mushrooms.
Place greens in a shallow bowl. Spoon mushrooms over top and serve as a starter or light meal.