Wash and slice the mushrooms. Saute mushrooms with the balsamic vinegar in a large skillet until fully cooked. Remove mushrooms from skillet and set aside. Keep the mushroom liquid in the skillet. Add 3 cups water, miso, tamari, and nutritional yeast to the skillet. Whisk until thoroughly blended and simmer for a few minutes. Add celery seed and black pepper to taste. Add mushrooms. Mix together cornstarch and quarter cup water in a cup. Make sure the cornstarch is fully dissolved. With skillet over medium heat, add the cornstarch suspension to the gravy a little at a time. Stir continually. When gravy reaches desired thickness, cook for a minute longer and then serve immediately. Note: If you want to make this ahead, make up the miso-mushroom mixture as described but do not add cornstarch. When you are ready to serve it, heat up the miso-mushroom mixture and thicken before serving. Makes 4 cups.