Cube the beef and then pat dry with paper towels. Generously salt and pepper all the sides.
Heat 1 tablespoon oil in a large skillet or pot over medium-high heat and once it's shimmering, add the beef cubes (in one even layer; you can do this in batches to not overcrowd the meat). Sear for about 15-20 seconds per side (about 1 minute total) and then transfer the beef to a large slow cooker.
Turn heat down to medium-low and add the remaining 2 tablespoons of olive oil. Add the onion and saute for 2 minutes. Add the garlic and saute for 30 seconds. Next, add in the carrots and celery and continue to saute, stirring occasionally for 5-8 minutes.
Turn the heat up to high, add 1 1/2 cups of beef stock (or red wine), and scrape any browned bits up from the pan. Stir and simmer for 3 minutes and then pour the veggie mixture into the crockpot.
While the veggies are simmering, you can add everything else to the crockpot: tomato paste, chopped potatoes, crumbled beef bouillon cubes, Worcestershire sauce, fresh thyme, bay leaves, uncooked pearl barley, white sugar, dried basil, dried oregano, undrained fire-roasted tomatoes, and finally the rest of the beef stock.
Stir to combine. Cover and cook on low for 7-8 hours or on high for 4-5 hours.
Remove bay leaves and taste for seasoning. It will vary greatly depending on the actual beef stock and bouillon cubes you use, but I generally add about 1/2 teaspoon salt and 1/4 teaspoon pepper.
Enjoy with fresh parsley (optional) and a crusty loaf of bread!