Step 1: Prepare the chicken. Pat the chicken breasts dry with paper towels. Season lightly with salt and pepper.
Step 2: Whisk the sauce. In a medium bowl, whisk together the soy sauce, honey, apple cider vinegar, minced garlic, grated ginger, and red pepper flakes (if using).
Step 3: Combine in slow cooker. Place the chicken breasts in the bottom of your slow cooker. Pour the sauce mixture evenly over the chicken.
Step 4: Cook. Cover and cook on LOW for 6-8 hours or on HIGH for 3-4 hours, or until the chicken is cooked through and easily shreds.
Step 5: Thicken the sauce. Once the chicken is cooked, carefully remove it from the slow cooker and set aside on a cutting board. Whisk the cornstarch slurry into the sauce remaining in the slow cooker. Turn the slow cooker to HIGH (if not already) and cook for another 15-20 minutes, stirring occasionally, until the sauce has thickened.
Step 6: Shred and serve. Shred the chicken using two forks, or slice it into desired pieces. Return the chicken to the slow cooker and toss to coat with the thickened sauce. Garnish with fresh parsley or green onions. Serve hot over cooked rice or quinoa with a side of steamed vegetables.