Prep the slow cooker: Lightly spray the insert or rub a bit of oil to prevent sticking.
Make the sauce: In a bowl, whisk honey, soy sauce, garlic, rice vinegar, ketchup (or tomato paste), sesame oil, ginger, and red pepper flakes until smooth.
Add the chicken: Place chicken in the slow cooker in a single layer if possible. Pour the sauce over the top, ensuring the chicken is coated.
Set and cook: Cover and cook on Low for 4–5 hours or High for 2.5–3 hours, until the chicken is tender and reaches 165°F.
Shred or slice: Remove chicken to a cutting board.
Shred with two forks or slice into chunks, then set aside.
Thicken the sauce: Whisk cornstarch and water to make a slurry. Stir it into the cooking liquid in the slow cooker. Cover and cook on High for 10–15 minutes, or until the sauce thickens and turns glossy.
Combine: Return the chicken to the slow cooker and toss in the sauce to coat evenly.
Taste and adjust: Add a splash more vinegar if it’s too sweet, or a drizzle of honey if you want it sweeter.
Season with a pinch of salt if needed.
Serve: Spoon over rice or noodles. Garnish with sliced green onions and sesame seeds.