In a large glass bowl, stir together chopped onions, tomato paste and thyme. Cook in microwave, stirring halfway through, until softened, about 5 minutes.
Place onion mixture, tomatoes, beef stock, carrots, soy sauce & pearl barley (everything but beef, parsley, salt & pepper) into slow cooker. Stir until combined.
Trim fat from beef and season with salt and pepper. Nestle beef into slow cooker and cook on low for 9-10 hours or on high for 6-7 hours.
Remove beef from slow cooker and shred with two forks. Skim extra fat off surface of the soup (if any), and return shredded beef to the soup. Stir.
Top with fresh parsley and season with extra salt and pepper, if needed.