Step 1: Optional Browning of Beef : Heat olive oil in a skillet over medium-high heat. Brown beef cubes in batches, about 3–4 minutes per side. This step adds extra flavor but can be skipped for convenience.
Step 2: Prepare Slow Cooker : Place beef, onion, garlic, carrots, and celery into the slow cooker. Add barley, beef broth, thyme, rosemary, bay leaves, salt, and pepper. Stir to combine.
Step 3: Slow Cook : Cover and cook on low for 6–8 hours or on high for 4–5 hours until beef is tender and barley is cooked. About 15 minutes before serving, stir in frozen peas if using.
Step 4: Season and Serve : Remove bay leaves. Adjust seasoning with salt and black pepper to taste. Ladle soup into bowls and garnish with fresh parsley.