Start by trimming excess fat from the chicken thighs and then chopping the onions and carrots into roughly 1-inch pieces. Rinse the barley under cold water until the water runs clear, then set aside.
Layer the chopped onions and carrots at the bottom of your slow cooker. Add the rinsed barley evenly over the vegetables. Nestle the chicken thighs among the ingredients, spreading them out so they cook evenly.
Pour the chicken broth over everything, making sure the ingredients are mostly submerged. Sprinkle the dried thyme on top and give everything a gentle stir to distribute the seasoning.
Cover the slow cooker with the lid and cook on low for about 6 to 8 hours. During this time, the barley will become plump and chewy, the chicken will turn tender and shred easily, and the flavors will meld into a savory, smoky aroma.
Once the cooking time is up, carefully remove the chicken thighs and shred the meat with two forks. Return the shredded chicken to the soup, stirring gently to combine all the ingredients evenly.
Squeeze fresh lemon juice into the soup to brighten the flavors, and stir in chopped parsley for a fresh herbal note. Taste and adjust seasoning with salt and pepper as needed.
Serve the soup hot, ladled into bowls with a sprinkle of extra parsley if desired. Enjoy the comforting, hearty textures and deep, smoky flavors in every spoonful.