Arrange the chicken pieces in a single layer at the bottom of the slow cooker.
Scatter the diced onion and minced garlic over the chicken.
In a small bowl, whisk together tamari/soy sauce, honey, rice vinegar, black pepper, and grated ginger.
Pour the sauce mixture over the chicken.
Cover the slow cooker and set it to LOW for 2 to 3 hours or HIGH for 1 to 2 hours.
Once cooked, transfer the chicken to a bowl using a slotted spoon.
Pour the sauce into a small saucepan and bring it to a boil.
Let the sauce reduce by half for about 15 to 20 minutes.
Whisk together the water and cornstarch, then stir it into the sauce until thickened.
Return the chicken to the slow cooker and coat it with the thickened sauce.
Serve the chicken over cooked rice and garnish with red pepper flakes, sesame seeds, and sliced scallions.
