Cook the mushrooms in a saucepan for around 5 minutes, until they are cooked through. 

Meanwhile, add the onion to a non-stick frying pan and sauté in either water or oil for a few minutes, allowing it to soften, and prepare the sauce by stirring together the tamari, balsamic vinegar, maple syrup, water, and cornstarch. 

Drain and rinse the mushrooms, transferring them to the frying pan together as well as the sauce. Stir over medium heat until the sauce thickens. Serve over cooked rice or noodles.