Heat the olive oil in a medium saucepan over medium heat. Add the onions and mushrooms and cook until softened, 8-10 minutes.
Sprinkle the flour and thyme over the mushroom mixture and stir until the flour is incorporated and the thyme is fragrant, about 1 minute.
Add the broth and soy sauce, and simmer until the sauce thickens, whisking occasionally, about 10-15 minutes. Remove it from the heat and serve.
Video
Notes
Storage: Store any leftovers in an airtight container. They will last about 5 days in the fridge.Make Ahead Tips: You can make the gravy up to five days ahead and store it in the fridge. Reheat just before serving. If the gravy thickens too much, you can add vegetable broth to thin it out.Substitutes: For best results, follow the recipe as is. However here are some common substitutes that would work well in this recipe.
You can use chicken broth or beef broth instead of vegetable broth (although it won't be vegan anymore)
You can substitute cornstarch, arrowroot starch, potato starch or tapioca for flour. Use half as much cornstarch as you would flour. Since the recipe calls for 2 tablespoons of flour, you would use only 1 tablespoon of cornstarch, for example
Equipment: If you prefer a smooth gravy without any onions or mushrooms chunk, I highly recommend using the Cuisinart Smart Stick hand immersion blender. It's quick and easy and will give you the perfect consistency.