Heat olive oil in a large pot over medium heat.
Add onion, carrots, and celery; sauté until softened.
Stir in garlic and mushrooms; cook until mushrooms release moisture.
Add barley, broth, bay leaf, and thyme.
Bring to a boil, then reduce heat and simmer.
Cook for 40–45 minutes until barley is tender.
Season with salt and pepper.
Stir in parsley and lemon juice before serving.