Heat the olive oil in a large pot or Dutch oven over medium heat.
Add the beef cubes to the pot and season with salt and pepper. Brown the beef on all sides, about 5-7 minutes.
Add the diced onion and minced garlic to the pot with the beef. Sauté for another 3-4 minutes until the onion is translucent.
Stir in the diced carrots, celery, and sliced mushrooms. Cook for an additional 5 minutes.
Add the barley, beef broth, bay leaves, and dried thyme to the pot.
Bring the mixture to a boil, then reduce the heat to low. Cover and let it simmer for about 1 hour, stirring occasionally.
After an hour, check the beef for tenderness. If it's not tender, continue to simmer for an additional 20-30 minutes.
Once the beef and barley are tender, remove bay leaves from the soup.
Adjust seasoning with salt and pepper as needed.
Serve the soup hot, garnished with chopped parsley.