In a large skillet over medium high heat, heat the olive oil. Add the onions and saute for 3 minutes.
Add the mushrooms and continue to cook on high for 5 minutes, more stirring occasionally. Onions should become clear and the mushrooms should give up their liquid and start to brown lightly. Season to taste with salt and pepper.
Add the balsamic vinegar and and herbs and stir, scraping up any browned bits. Cook 5 minutes more on medium heat until the liquid has evaporated.