Measure out the miso and let sit at room temperature.
Heat a large, wide frying pan over medium heat. Add 1 ½ tablespoons (21g) of the butter (or olive oil). Once melted or hot, add the diced onions with a pinch of salt. Sauté until translucent and golden, about 5 minutes.
Cook the mushrooms: Increase the heat to medium-high and add the mushrooms, spreading them out as much as you can. Cook for 5 minutes, stirring occasionally, or until they release their liquid and soften (stir more frequently if not using a non-stick pan).Add the remaining 1 ½ tablespoons (21g) butter (or olive oil) and stir well to coat the mushrooms. Stir frequently until mushrooms are somewhat browned, about 3 minutes.Multitasking: While the mushrooms cook: (a) in a large measuring cup or bowl, combine 2 cups (480 mL) of the broth, the red wine, and soy sauce; (b) make the “roux”: in a separate glass, whisk together the remaining ¼ cup (60 mL) broth, the nutritional yeast, and cornstarch To the pan, add the garlic, thyme, rosemary, ¼ teaspoon kosher salt, and several cracks of pepper. Cook for 1 minute, stirring almost constantly, or until very fragrant.
Pour in some of the red wine mixture and scrape up any browned bits, then pour the entire mixture in. Bring to a boil and boil for 3 to 4 minutes to cook off the wine, stirring occasionally.
Re-whisk the “roux” to combine, then add to the pan. Whisk constantly for 2 minutes to dissolve and thicken, lowering the heat if it bubbles vigorously.
Reduce the heat to medium and add in the miso. Stir very well to incorporate, breaking it up with your spatula. Simmer for 3 to 4 minutes, stirring very frequently, until the gravy has thickened.
Remove from the heat. Stir in ¼ teaspoon balsamic vinegar, then taste, adding more as needed. Season with black pepper to taste.NOTE: For a smoother consistency, scoop out 2 ladles of gravy into a blender or food processor. Blend until smooth. Add the blended gravy back to the pan and mix together.