20 ounces button mushrooms stems trimmed and caps thinly sliced
3 14.5-ounce cans low-sodium chicken or vegetable broth
2 bay leaves
8 sprigs fresh thyme
Sourdough or some other bread toasted (optional)
Bring the barley and 4 cups of water to a boil in a medium saucepan. Reduce heat, cover, and simmer until tender, 30 to 40 minutes. Meanwhile, heat the oil in a large pot over medium-low heat. Add the onions, salt, and pepper and cook for 7 minutes. Add the carrot and celery and cook, stirring occasionally, for 6 minutes more. Add the mushrooms, increase heat to medium-high, and cook until they release their juices, about 3 minutes. Add the broth, bay leaves, and thyme and simmer for 10 minutes. Stir in the cooked barley and cook for 5 minutes more. Remove and discard the bay leaves and thyme sprigs. Ladle the soup into individual bowls and serve with the toasted bread, if desired.
To freeze: Let the soup cool, then ladle it into resealable freezer bags. Store for up to 3 months.
To reheat: Thaw overnight in the refrigerator or thaw partially in the microwave. Warm in a covered saucepan over medium-low heat for 20 minutes. Serve with the toasted bread, if desired.