01 - Place the chicken breasts in an even layer across the bottom of the slow cooker.
02 - In a medium bowl, whisk together the brown sugar, minced garlic, soy sauce, chicken broth, apple cider vinegar, dried thyme, black pepper, smoked paprika, and red pepper flakes until well combined.
03 - Pour the sauce mixture evenly over the chicken breasts. Cover the slow cooker and cook on low for 4 to 5 hours, or until the chicken is cooked through and tender, reaching an internal temperature of 165°F.
04 - For a thicker sauce, remove the chicken and transfer the sauce to a saucepan. Simmer for 5 to 10 minutes until reduced. Alternatively, stir 1 tablespoon of cornstarch mixed with 1 tablespoon of cold water into the slow cooker sauce and cook on high for 15 minutes.
05 - Serve the chicken hot, spooning the sauce generously over the top. Garnish with chopped fresh parsley or sliced green onions if desired.