Rinse the pearl barley under cold water and set aside. Peel and dice the carrots. Dice the celery stalks. Dice the large onion. Mince the garlic cloves. Chop the mushrooms.

Place the boneless, skinless chicken breasts at the bottom of the slow cooker. Add the rinsed pearl barley on top of the chicken. Add the diced carrots, celery, onion, and minced garlic to the slow cooker. Pour in the 8 cups of low-sodium chicken broth. Add salt and pepper to taste, along with the 1⁄2 teaspoon of black pepper. Add the chopped mushrooms to the slow cooker.

Cover the slow cooker with its lid. Cook on low for 6-7 hours, or on high for 3-4 hours, until the chicken is fully cooked and the barley is tender.

Once cooked, remove the chicken breasts from the slow cooker. Shred the chicken using two forks and return it to the slow cooker. Stir to combine all the ingredients evenly. Taste and adjust the seasoning with additional salt and pepper if needed.

Ladle the soup into bowls and serve hot. Enjoy your hearty and comforting Slow Cooker Chicken Barley Soup!