In a 6 or 8 quart pot, heat oil. Add onions, carrots, celery, and mushrooms and cook until softened and onions become translucent. Add tomato paste, thyme, salt, and pepper. Cook for 2-3 more minutes then add stock and rinsed barley/brown rice into the pot. When soup comes to a boil, turn it down to simmer. Cook uncovered for about 1 hour or until barley/rice is tender.