Heat the olive oil in a large pot or Dutch oven over medium-high heat. Season the beef with salt and pepper, then brown it on all sides in batches. Do not overcrowd the Dutch oven. Remove and set aside.
Add the onion, carrots, celery, and mushrooms (if using) to the same pot. Sauté for 5-7 minutes until softened. Add the garlic and cook for 1 more minute until fragrant.
Return the browned beef to the pot. Stir in the beef broth, tomato paste, Worcestershire sauce, thyme, rosemary, bay leaf, and the rinsed pearl barley.
Bring the soup to a boil, then reduce the heat to low. Cover and simmer for 1 to 1.5 hours, or until the beef is tender and the barley is cooked through.
Remove the bay leaf. Taste and add more salt and pepper if needed. Garnish with fresh parsley and serve hot. Salt and black pepper to taste
Optional: Add 1 tsp unflavored gelatin to 1 TBS water and let it sit 5-10 minutes to bloom. Stir the gelatin in the soup and simmer for 10 minutes.