Heat the olive oil in a large pot or Dutch oven over medium heat.
Add the beef cubes to the pot, seasoning with salt and pepper. Brown the beef on all sides, about 5-7 minutes.
Once the beef is browned, remove it from the pot and set aside. In the same pot, add the onions, carrots, and celery. Sauté for about 5 minutes until the vegetables start to soften.
Add the minced garlic, thyme, and bay leaves to the pot, stirring well for about 1 minute until fragrant.
Return the browned beef to the pot, then add the beef broth, water, and pearl barley. Stir to combine.
Bring the mixture to a boil, then reduce the heat to low. Cover and let simmer for about 75 minutes, or until the beef is tender and the barley is cooked through.
Stir occasionally, and during the last 10-15 minutes of cooking, taste and adjust seasoning with salt and pepper as needed.
Once done, remove the bay leaves. Ladle the soup into bowls and garnish with fresh parsley if desired.