Heat the oil in a saucepan over medium heat.
Cook the mushrooms and onion for 5 minutes or until the moisture from the mushrooms has evaporated.
Add the minced garlic and cook for another 1 to 2 minutes until fragrant.
In a jar with a tight-fitting lid, combine 1 cup (250 mL) of the cold vegetable stock, flour, tamari (if using), nutritional yeast, thyme, and sage. Shake vigorously until the mixture is smooth and well blended.
Pour the flour mixture into the saucepan along with the remaining 3 cups (720 ml) of stock.
Stir everything together.
Bring the gravy to a boil, then immediately reduce the heat to low.
Cover and let it simmer for 15 to 20 minutes, stirring frequently to prevent sticking.
Stir in the fresh parsley, season with salt and pepper to taste, and serve warm.