Heat a large nonstick saucepan or Dutch oven. Swirl in the oil, then add the onions, carrots, and celery. Sauté until wilted, about 5 minutes. Add the wine; simmer until the liquid is reduced by one-fourth. Add the broth; bring to a steady simmer. Stir in the barley; cover and cook, stirring occasionally, until the barley is tender, about 45 minutes.
Meanwhile, clean and trim the mushrooms; separate the stems from the caps. Chop the stems and one-third of the caps; slice the remaining caps.
Transfer 1 1⁄2 cups of the vegetables and barley and 1⁄2 cup of the liquid to a food processor; puree. Return the mixture to the saucepan.
Add the mushrooms, parsley, and pepper; stir and bring to a gentle boil. Simmer, covered, until the mushrooms are cooked, 15 minutes. Garnish with the additional parsley, if using. Yields 1 1/2 cups per serving.
