Place a large non-stick skillet over medium heat and add 1 tbsp of olive oil, Add the sliced mushrooms and onions to the pan. After 2-3 minutes, add 1 tbsp of balsamic vinegar. Stir. Let it cook down for another 2-3 minutes and then add the second tbsp of Balsamic vinegar. Sauté for an additional 2-3 minutes, set aside.