Place the chicken thighs in the slow cooker.
In a bowl, stir together the honey, soy sauce, minced garlic, rice vinegar, and black pepper until smooth.
Pour the sauce over the chicken and turn the pieces to coat evenly.
Cover and cook on low for 5 to 6 hours until the chicken is tender.
In a small cup, mix the cornstarch and water to form a slurry.
Stir the slurry into the slow cooker and cook on high for 15 more minutes until the sauce thickens.
Serve the chicken with the sauce spooned over the top and sprinkle with sliced green onions.