Add the butter to a medium saucepan. Heat over medium heat until butter has melted and small bubbles form. Add the mushrooms, thyme, garlic powder and onion powder. Sauté until mushrooms have released their liquid, about 5 minutes.
Add the red wine, and scrape any bits off the bottom of the pan using a wooden spoon. Allow the wine to simmer until it reduces in volume by half. Add 1 ½ cups of the vegetable broth and bring it back to a simmer.
Whisk together the miso paste and ½ cup of the vegetable broth in a small bowl until the miso has dissolved. Add the miso mixture to the saucepan.
Whisk together the cornstarch and 2 tablespoons of water in a small bowl to make the slurry. Add the cornstarch slurry to the saucepan. Continue whisking the gravy for a couple minutes until it begins to thicken.
Turn off the heat. Enjoy as is, strain out the mushrooms or blend them usings an immersion blender.
Notes
You may use an assortment of mushrooms such as shiitake, oyster or white mushrooms.
Rapidly whisk the cornstarch slurry into the gravy so there aren’t gooey clumps.
Strain out the mushrooms for a smoother gravy, or blend the gravy with an immersion blender.
Double check that the miso paste is gluten-free (if necessary).