Start by chopping the carrots into thin slices and dicing the onion into small pieces. Pat the chicken thighs dry and lightly season with salt and pepper.
Place the chicken thighs at the bottom of the slow cooker, then layer the sliced carrots, diced onion, and rinsed pearl barley on top.
Pour the chicken broth over the ingredients until they are just covered, about four cups. Sprinkle the fresh thyme evenly over the top.
Cover the slow cooker and cook on low for 6 to 8 hours, until the chicken is tender and easily shreds, and the barley is plump and chewy.
Once cooking is complete, remove the chicken thighs and shred the meat into bite-sized pieces with two forks. Return the shredded chicken to the pot and stir to combine.
Squeeze the lemon juice into the soup and stir well. Taste and adjust seasoning if needed, then remove the thyme sprigs and bay leaf.
Let the soup sit off the heat for about 5 minutes to allow flavors to meld, then ladle into bowls and enjoy a warm, hearty bowl of comfort.