1. Preheat oven to 425 degrees Fahrenheit (220 degrees Celsius).
2. Toss onions and mushrooms with soy sauce, sesame oil, and one tablespoon balsamic vinegar until well coated.
3. Spread on a Gefen Parchment Paper-lined baking sheet. Bake for 15–20 minutes, until tender.
4. Remove from oven and sprinkle with remaining half tablespoon balsamic vinegar. Toss and serve.
To keep warm for Shabbat, cover and place in a 200–250 degrees Fahrenheit (90–130 degrees Celsius) oven until ready to serve, or place on a top of a blech or slow cooker to keep moist and warm.