In a small bowl, combine warm water with miso and mushroom powder. Stir to dissolve miso and set aside.
Heat olive oil in a medium saucepan over medium heat. Add garlic and onions, cooking until softened and stirring frequently. Turn up the heat to medium high, add mushrooms and cook until lightly browned and softened. At this point, most of the water from the mushrooms should cook off.
Add the red wine and balsamic vinegar cooking until the liquid evaporates. Add the cornstarch to the miso mushroom water and whisk until smooth. Add to pan, stirring to combine. Bring to a low boil and cook for two minutes. Sauce will thicken as it cooks. Taste and add salt and pepper as desired.
Stir in parsley just before serving.
Tips for BEST results:Mix in the cornstarch to the liquid before adding to the pan. Adding the cornstarch directly to the pan likely results in a lumpy sauce.For more mushroom flavor and meatier mushrooms use cremini mushrooms or portobello mushrooms. To reheat the sauce, heat over medium heat and add some additional water to thin it out if needed.