In a large pot or Dutch oven, heat the oil over medium-high heat until shimmering. Add the onion, celery, carrots, and a pinch of salt, and cook for 5 minutes, until the onion is translucent. Add the mushrooms and cook for an additional 10 minutes, until the mushrooms have released their liquid and most of it has evaporated. Add in the garlic, and sauté for another minute or so, until fragrant.
Stir in the tamari and dry sherry, then add the salt, pepper, vegetable stock or water, and barley. Bring to a boil, then reduce heat, cover, and allow to simmer for 45 minutes, until the vegetables and barley are very tender. Taste, adjust seasonings as necessary and serve. Leftover soup will keep in the refrigerator for up to 5 days.