How to Make It
Heat the olive oil and butter in a large skillet over medium high heat. Add the onions and cook until they turn a deep brown, stirring often, about 10-15 minutes. Add the garlic and mushrooms, reduce the heat to medium, and cook until the mushrooms have given up most of their liquid and the mixture is deeply fragrant, 10-15 minutes. Add the wine to the skillet and scrape any crispy bits from the pan. Stir in the nutritional yeast.
Add the stock and bring to a boil, then reduce the heat to low and simmer, uncovered, for 10 minutes. Add a pinch of salt and a few grinds of black pepper. (Just add a small amount of salt now to distribute the flavor; you’ll finish seasoning the gravy at the end, after adding the soy sauce.)
Pour the mixture into a blender and blend until smooth. Transfer to a bowl. Stir in two tablespoons of the soy sauce. If you want the gravy to be a darker brown, add another tablespoon or two. Taste and season with salt.