Place 1/2 cup of vegetable broth in a small saucepan over medium heat. Bring to a quick boil then, turn off the heat. Stir in the dried mushrooms. Cover the pot and allow to rest for 15 minutes.
In a large skillet or rimmed pan, heat the oil over medium-high heat. Add the onions and cook for about 2-3 minutes then, add the mushrooms and cook, stirring frequently for about 6-8 minutes or until golden brown. Stir in the garlic and cook, for about a minute or until aromatic.
Make Roux: Sprinkle in the flour while stirring constantly. Cook, stirring for about 2 minutes.
Slowly, stir in the rehydrated mushrooms and their liquid and whisk in 4 cups of broth a little at a time until a smooth sauce forms.
Season: Add the thyme and rosemary (if using) and simmer for about 3 minutes. Stir in the soy sauce, miso, nutritional yeast and ground black pepper to taste.
Blend: Transfer the gravy to a blender or use an immersion blender and blend until smooth (you can also leave a few chunks of onion and mushrooms for texture if preferred.
Return to the Pan: Return the gravy to the pan. Add the reserved broth if needed to reach the desired consistency. Adjust seasoning to taste and simmer for about 2-3 minutes. Serve.