Bring a large pot of generously salted water to a boil. While you are waiting for the water to boil, heat the olive oil in a skillet on medium high heat and add the sliced mushrooms. Season with salt and pepper. Cook for 5-8 minutes. Add the pearl onions, season with more salt and pepper and cook for another 5 minutes or so, until both the mushrooms and the onions are golden. Add the shallots and the garlic. Cook for 3-5 minutes more. Add the balsamic vinegar, scraping the bottom of the skillet to get all the browned bits loosened. This will add a lot of flavor to the dish. If your skillet still has a lot of brown bits, pour in a bit of water or broth.

When the water has boiled, add the green beans and cook for about 3 minutes, just until the beans are tender but still vibrant green and crisp. Drain the green beans. Add the green beans to the skillet with the mushrooms. Mix to combine. Serve immediately. You can reheat the green beans very well in a skillet.