From The Happy Herbivore
Whisk nutritional yeast and flour together with plant-based milk and set aside.
Line a large frying pan with a thin layer of water and cook onion and mushroom over high heat until the mushrooms start to soften and turn brown and most of the water has evaporated.
Add plant-based milk mixture, soy sauce, and herbs, stirring to combine.
Bring to a boil and continue to cook until thick. Add salt and pepper to taste.