Pat beef dry with paper towels and season with salt and pepper. Heat 1 tablespoon olive oil in a large Dutch oven over medium-high heat.

Working in batches, brown the beef on all sides until it develops a deep brown crust, about 5-7 minutes per batch. Add a little more oil between batches if needed. Avoid overcrowding the pot. Transfer the beef to a plate.

Reduce the heat to medium and add the remaining tablespoon of olive oil. Add onion, carrots, and celery. Cook for 7-8 minutes, stirring occasionally, until softened.

Add sliced mushrooms and cook for 6-8 minutes, stirring occasionally, until they have released most of their moisture and begin to brown.

Stir in garlic and cook for 30 seconds, until fragrant. Add tomato paste and mustard. Cook for 1 minute, stirring constantly, until the tomato paste darkens slightly.

Pour in red wine and scrape up all the browned bits from the bottom of the pot. Simmer until the wine has reduced by about half, about 3-5 minutes.

Return the browned beef and any juices to the pot. Add beef broth, pearl barley, bay leaves, thyme, rosemary, and Worcestershire sauce.

Bring to a boil and reduce the heat to low. Cover partially and simmer for about 80 minutes, stirring occasionally, until the beef is fork-tender and the barley is cooked through. If the soup becomes thicker than you'd like, stir in a splash of additional beef broth or water.

Remove the bay leaves. Stir in red wine vinegar and parsley. Serve with crusty bread.