Instructions:
- Pat the chicken pieces completely dry with paper towels. Place chicken evenly in the bottom of the slow cooker insert.
- In a medium bowl, whisk together the soy sauce, water, brown sugar, rice vinegar, mirin, grated ginger, minced garlic, and sesame oil until the sugar is mostly dissolved.
- Pour the entire sauce mixture over the chicken in the slow cooker. Cover and cook on LOW for 7-8 hours or on HIGH for 3-4 hours, until the chicken is fork-tender.
- Carefully remove the cooked chicken from the slow cooker and place it on a cutting board. Use two forks to shred the meat. Set aside.
- Create the cornstarch slurry by whisking the cornstarch and 1/4 cup of cold water together in a small bowl until completely smooth.
- Turn the slow cooker setting to HIGH (if it wasn't already) and let the remaining liquid come to a simmer. Slowly drizzle the cornstarch slurry into the simmering sauce while whisking constantly.
- Allow the sauce to simmer for 3–5 minutes, whisking occasionally, until it thickens to a glossy, syrupy glaze consistency.
- Return the shredded chicken to the pot. Toss well to coat every piece thoroughly in the thick teriyaki glaze.
- Serve immediately, garnished generously with toasted sesame seeds and sliced spring onions.