Preheat oven to 350°F (175°C).
Heat olive oil in a large oven-safe pot over medium heat.
Add onion, carrots, and celery. Cook for 5 minutes.
Add mushrooms and cook until softened, about 5 minutes.
Add barley, broth, tomatoes, and herbs.
Bring to a boil, then cover and transfer to oven.
Bake for 45 minutes until barley is tender.
Stir in peas and bake 5 minutes more.
Season with salt and pepper before serving.