Keep the beef chuck at 40°F or colder while you work. Cut it into irregular 1-inch cubes during the first 5 minutes. Then slice the carrots into 1/2-inch coins and celery into 1/2-inch pieces. Dice the yellow onion and cut Yukon Gold potatoes into 3/4-inch cubes. Mince the garlic last. Finish the cutting within 12 to 15 minutes. The beef should look firm and deep red with clean cut surfaces. The vegetables should remain crisp and brightly colored. You'll smell fresh onion and garlic once they're cut. Keep the potato cubes close in size but don't chase perfect shapes. Oversized pieces stay firm while tiny pieces can disappear during the long cook. Consistent cutting helps each ingredient soften at roughly the same rate.

Pat the beef cubes dry and season them with 1 teaspoon kosher salt and 1/2 teaspoon black pepper. Heat 1 tablespoon extra-virgin olive oil until the cooking surface reaches about 425°F. Add half the beef in one loose layer. Sear 2 to 3 minutes per side and turn each cube with steady motions. Repeat with the remaining beef for about 10 minutes total. Look for a dark amber crust along several faces while the centers remain red. You'll hear a strong steady sizzle and smell roasted beef as the crust develops. If liquid collects instead of sizzling, the pan is overcrowded. That prevents proper browning and weakens the broth's flavor. (I used to crowd every cube together, the beef steamed instead of forming that dark crust.) High heat drives the Maillard reaction that builds the soup's savory base.

Lower the cooking surface to about 325°F and add the remaining 1 tablespoon extra-virgin olive oil. Add diced yellow onion and celery. Stir and scrape across the surface every 45 seconds for 5 to 6 minutes. Add minced garlic and tomato paste. Stir continuously for 60 to 90 seconds so the paste spreads across the softened vegetables and darkens one shade. The onion should turn translucent while celery edges soften but keep their pale-green color. The tomato paste should change from bright brick red to a darker rust color. You'll smell toasted tomato and garlic right before it's ready. Stop if the garlic turns brown because burnt garlic makes Crockpot Beef Barley Soup bitter. Toasting the tomato paste removes raw acidity and concentrates savory glutamates.

Transfer the browned beef and toasted aromatic mixture into a 6-quart slow cooker. Add carrot coins and Yukon Gold potato cubes. Pour in low-sodium beef broth and Worcestershire sauce. Add dried thyme and bay leaves. Stir twice from bottom to top so everything sits partly submerged. Cook Crockpot Beef Barley Soup on LOW at about 200°F for 6 1/2 to 7 hours. By hour 6 1/2, the broth should look deep reddish brown and small beads of beef fat should glisten on top. The carrots soften while keeping their round shape and the potatoes become creamy at their edges. The beef should separate easily under gentle pressure and reach roughly 195°F to 205°F for braise-like tenderness. Cooking far beyond 8 hours can make the vegetables collapse. Slow moist heat converts chuck collagen into gelatin without violently breaking the vegetables apart.

Stir rinsed pearl barley into the hot broth and switch the slow cooker to HIGH. Keep the liquid near a gentle 200°F simmer for 45 to 60 minutes. Stir once around minute 30 and scrape through the center so the grains redistribute. Remove both bay leaves when the barley feels tender but still slightly chewy. Stir in half the chopped flat-leaf parsley. The pearl barley should look plump and pale tan while the broth becomes slightly thicker and glossy. Beef pieces should remain irregular and fork-tender. Carrots and potatoes should hold their shapes with softened edges. Go much past 60 minutes and the barley can swell excessively and absorb too much broth. Ladle one portion into a white bowl and scatter the remaining parsley unevenly over it. Controlled finishing time keeps the barley separate and pleasantly chewy.