Combine olive oil and vinegars in small saucepan on stove.

Mince garlic and chop green onion fairly small.

Whisk onions and garlic into balsamic mixture.

Clean mushrooms and remove stems.

Add mushrooms to saucepan and slowly bring to a boil.

Stir gently to make sure all mushrooms get coated. Once boiling, reduce heat and let the mushrooms simmer until sauce has reduced and mushrooms are cooked through. (About 15 mins.)