In a large Dutch oven, heat oil over medium-high heat. Season beef with salt and pepper, to taste. Working in batches, when the oil shimmers, add the beef and brown on all sides (it doesn’t need to be cooked through – just make sure it gets nice color on the outside) – about 10 minutes. Use a slotted spoon to remove the meat from the pot and set it aside (leave the fat in the pot).
Add the carrots, celery and onion to the pot. Cook, stirring occasionally, for 10 minutes. Add mushrooms and garlic and cook, stirring, for one more minute. Stir in the tomatoes, thyme, bay leaves and broth. Scrape up all of the flavorful beef bits from the bottom of the pot as you stir! Return the beef to the pot. Season with ½ teaspoon salt and ¼ teaspoon pepper.
Increase the heat and bring soup to a boil. As soon as the soup boils, reduce the heat to low, cover, and simmer for 1 hour (or until beef is tender). Add the quick barley, cover, and simmer for 10-15 more minutes (or until barley is tender).
Taste the soup and season with additional salt and pepper, if needed.
Ladle soup into bowls and garnish with fresh herbs, if desired.