In a large stock pot, heat the oil to medium-high heat. Add the onions, mushrooms, carrots, thyme, pepper and garlic. Saute until the onion is translucent, about 5 minutes.
Add the vegetable stock and barley and bring to a boil. Reduce heat and simmer for about 20 minutes or until barley is tender. Stir in sherry and potato. Continue to simmer until potato is cooked, about 15 minutes. Garnish with sliced green onions and serve.