Place chicken thighs in a 5 to 7-quart slow cooker.
In a mixing bowl, whisk together ketchup, honey, soy sauce, sriracha, sesame oil, garlic, and ginger. Pour the mixture evenly over the chicken thighs in the slow cooker.
Cover and cook on low heat until the chicken is cooked through and tender, approximately 3 to 4 hours (reaching 165 °F in the center).
Move the cooked chicken to a serving plate, and cover it with foil to keep it warm.
Pour the cooking juices from the slow cooker into a medium saucepan (you should have 2 cups of liquid), add vinegar, and bring to a simmer over medium heat.
In a small mixing bowl, whisk together cornstarch with 2 tablespoons of water until well blended. Pour the mixture into the simmering sauce.
Simmer the sauce, whisking constantly, for 1 minute.
Pour the sauce over the chicken or baste the chicken with it. Garnish with green onions and sesame seeds if preferred.