Heat the olive oil in a large frying pan over medium high heat. Add the onions and cook about 4 minutes, or until starting to soften. Add the garlic, mushrooms, thyme, and rosemary and black pepper and cook until the mushrooms reduce in volume and the liquid has mostly but not fully evaporated. Add the 1/2 teaspoon kosher salt and cook for 1 more minute.
Add the arrowroot and whisk it in to coat the mushrooms, whisking constantly for 1 minute. Pour in the vegetable broth and bring to a simmer, whisking until thickened, about 2 minutes.
Transfer the gravy to a blender and add the miso paste. Blend until the gravy is creamy and smooth.
Transfer the gravy back to the pan and heat over low heat for 3-5 minutes to further thicken it up. Taste and adjust seasonings as needed.
Store leftover cooled gravy in an airtight container in the fridge for 4-5 days, or in the freezer for a month.