Heat olive oil in a large skillet over medium-high heat. Brown the beef cubes in batches, ensuring each piece gets a nice sear. This step is crucial for developing deep, rich flavors in your soup.
Transfer the browned beef to your slow cooker.
In the same skillet, sauté the onions, carrots, celery, and garlic until slightly softened, about 5 minutes. Add these to the slow cooker.
Pour in the beef broth and diced tomatoes. Add the barley, bay leaves, thyme, salt, and pepper.
Give everything a good stir, cover the slow cooker, and set it to low for 8 hours or high for 6 hours.
About 30 minutes before serving, taste and adjust the seasoning if needed.