1. Prepare the Vegetables: Start by chopping your onion, dicing the carrots and celery, and mincing the garlic. This ensures all your aromatics are ready to go.

2. Combine Ingredients in Slow Cooker: Place the boneless, skinless chicken breasts or thighs at the bottom of your slow cooker. Add the rinsed pearl barley, chopped onion, diced carrots, diced celery, and minced garlic over the chicken.

3. Add Liquids and Seasonings: Pour in the low-sodium chicken broth and the can of undrained diced tomatoes. Sprinkle in the dried thyme, dried rosemary, black pepper, and salt. Give everything a gentle stir to combine the ingredients evenly.

4. Set and Cook: Cover your slow cooker with its lid. Cook on the LOW setting for 8-10 hours or on the HIGH setting for 4-6 hours. The soup is ready when the chicken is tender and easily shredded, and the barley is soft and fully cooked.

5. Shred the Chicken: Once cooked, carefully remove the chicken breasts or thighs from the slow cooker. Using two forks, shred the chicken into bite-sized pieces. Return the shredded chicken to the slow cooker and stir it into the soup.

6. Add Optional Peas and Finish: If using, stir in the frozen peas during the last 30 minutes of cooking on either setting. This ensures they are cooked through but still retain their vibrant color and texture.

7. Adjust Seasoning and Serve: Taste the soup and adjust salt and pepper if needed. Ladle the warm soup into bowls and garnish with fresh chopped parsley before serving.