Season the beef. Pat the beef dry, then season generously with salt and pepper. Dry surfaces sear better and build flavor.
Sear for depth (optional, but worth it). Heat olive oil in a skillet over medium-high.
Brown the beef in batches, 2–3 minutes per side. Don’t crowd the pan. Transfer browned beef to the crockpot.
Bloom the aromatics. In the same skillet, add onion, carrots, and celery with a pinch of salt.
Cook 4–5 minutes until softened and lightly browned. Stir in garlic and tomato paste for 30 seconds until fragrant. Scrape everything into the crockpot, deglazing the skillet with a splash of broth if needed.
Load the cooker. Add rinsed barley, beef broth, Worcestershire, thyme, rosemary, bay leaves, and any optional mushrooms or potatoes.
Stir to combine.
Set and cook. Cover and cook on Low for 7–9 hours or High for 4–5 hours, until beef is very tender and barley is plump.
Finish and adjust. Remove bay leaves. If using peas, stir them in during the last 10 minutes. Taste and adjust salt and pepper.
Add a splash of water or broth if the soup is thicker than you like.
Serve. Ladle into bowls, top with chopped parsley, and serve with crusty bread or a simple green salad.