01 - Place the chicken breasts in an even layer across the bottom of the slow cooker.
02 - In a medium mixing 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 reaches an internal temperature of 165°F and is fork-tender.
04 - For a thicker sauce, remove the chicken and transfer the sauce to a saucepan. Simmer over medium heat for 5 to 10 minutes until reduced to desired consistency. Alternatively, stir 1 tablespoon of cornstarch mixed with 1 tablespoon of cold water into the sauce in the slow cooker 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.