In a medium skillet over medium high heat, add mushrooms and onion. Stir occasionally until onion is soft and translucent, about 8-11 minutes. Add garlic powder and remove from heat.
Meanwhile, to a high powered blender, add raw cashews and nutritional yeast. Blend until of fine consistency. Add vegetable broth, tamari, thyme, rosemary, and black pepper. Add mushrooms/onions when ready. Blend slow at first, then increase the speed to medium until blended. Add vegetable broth (in 1/4 cup increments) if gravy is too thick and blend again.
Makes 2 cups gravy. Serving size 1/4 cup. You can store this in an airtight container in the fridge for 4-5 days. This gravy is great mixed in with pasta, polenta, lentils, rice, etc. =)