Heat a large skillet over medium high heat and add the olive oil.
Add the onions and saute until translucent, about 3-5 minutes. Add the mushrooms and saute until cooked down, about 3-5 minutes. Add the garlic and cook for another minute.
Add ¼ cup vegetable broth and the flour and whisk vigorously.
Follow with the rest of the broth, milk, salt, pepper, garlic powder, chopped sage, and the thyme and rosemary sprigs.
Allow the gravy to come to a simmer, whisking frequently while it thickens.
Once thickened, allow the gravy to simmer on low heat for 5-10 minutes for flavors to infuse. Remove from the heat and take out the thyme and rosemary sprigs from the pan before serving.