Place the dried shiitake mushrooms in a small bowl and cover with ½ cup of boiling water. Let them soak for at least 15 minutes to rehydrate. Remove the mushrooms, then strain the soaking liquid through a fine-mesh sieve or cheesecloth. Finely chop the rehydrated mushroom.
In a medium saucepan or skillet, melt 1 tablespoon of butter. Add the onion/shallots and fresh mushrooms. Season with salt. Cook for 8-10 minutes, stirring occasionally, until the mushrooms have released their liquid and are browned.
Stir in the rehydrated/chopped mushroom for the last minute. Cook for a minute. Transfer all the mushrooms with onion to a plate.
To make roux, heat vegetable oil in the same pan. Saute garlic for 20 – 30 seconds. Sprinkle the flour. Cook for 1-2 minutes, stirring constantly, to cook out the raw flour taste.
Slowly whisk in the reserved Shiitake liquid and the vegetable stock, stirring until no clumps of flour remain. Bring the gravy to a simmer over medium heat. Reduce the heat to low and let it cook, stirring occasionally, for about 5 – 10 minutes, or until it has thickened to your desired consistency.
Add the cooked mushrooms and stir to combine. Remove the pan from the heat. Stir in the remaining butter for extra richness and shine.
Season with salt and freshly ground black pepper, thyme to taste. If using, stir in the optional dry sherry or red wine vinegar for an extra layer of complexity.
Serve mushroom gravy hot over your favourite steak, roast, or side dish!