In a bowl, whisk together the honey, soy sauce, rice vinegar, minced garlic, ground ginger, and black pepper until well combined.
Place the chicken thighs in the slow cooker and pour the honey garlic mixture over them, ensuring the chicken is well coated.
Cover the slow cooker and cook on low for 4 hours, or until the chicken is tender and fully cooked.
Once cooked, remove the chicken from the slow cooker and place it on a cutting board.
In a separate bowl, mix the cornstarch with water to create a slurry.
Pour the remaining sauce from the slow cooker into a saucepan and bring to a simmer over medium heat.
Stir the cornstarch slurry into the sauce and cook, stirring constantly, until the sauce has thickened, about 2-3 minutes.
Slice the chicken and return it to the slow cooker, mixing it with the sauce.
Serve the honey garlic chicken over cooked rice and garnish with chopped green onions if desired.