Add vegan butter to a frying pan or saucepan and melt over medium heat. (See note below about using water or vegetable stock to make this oil-free.)
Sauté onion and garlic with a tiny pinch of sea salt for about 5 minutes. Stir often to ensure that the garlic does not burn.
Once the onions have softened, add the mushrooms and continue to cook for another 8 minutes.
Add nutritional yeast and flour, stir to evenly coat the mushrooms and onions.
Stir in vegetable stock and soy sauce or tamari. Continue to stir, breaking down any clumps. Allow the gravy to cook on medium-low heat, until it thickens up, stirring often. You can add a couple of tablespoons of stock or water if you need to thin out the gravy.
Add fresh cracked pepper and more salt, if desired, to taste.
Optional: blend for a smoother gravy. This was suggested in a comment by someone who blended it and I’ve since blended it myself and loved the result!