Heat 1 tablespoon oil in a wide 5- to 6-quart Dutch oven over medium-high heat. Add half the mushrooms and cook 5–7 minutes, stirring occasionally, until moisture evaporates and edges brown. Transfer to a bowl. Repeat with remaining oil and mushrooms.

Return all mushrooms to the pot. Add onion, carrot and celery. Cook 6–8 minutes, stirring, until onion softens; add a splash of the measured water if the pot is too dry. Stir in garlic for 30 seconds.

Add barley, broth, remaining water, thyme, salt and pepper. Scrape up browned bits. Bring to a boil, then reduce heat and simmer partly covered for 30–40 minutes, stirring occasionally, until barley is tender with a pleasant chew. Follow package guidance if it needs longer.

Add water as needed for a ladleable soup. Taste and divide into six servings. Refrigerate leftovers in shallow containers within 2 hours, or 1 hour above 90°F, at 40°F or below. Use within 3–4 days. Reheat throughout to 165°F, loosening with water as needed.