Whisk the sauce. In a bowl, combine soy sauce, honey, rice vinegar, sesame oil, garlic, ginger, and chicken broth. Taste and adjust sweetness or acidity if needed.
Load the crockpot. Place the chicken in the slow cooker. Add onion or pineapple if using.
Pour the sauce over the top, making sure everything is coated.
Cook low and slow. Cover and cook on Low for 4–5 hours or High for 2.5–3 hours, until the chicken is tender and easily shreds.
Shred or slice. Transfer the chicken to a cutting board and shred with two forks or slice into chunks. Leave the cooking liquid in the crockpot.
Thicken the sauce. In a small bowl, stir cornstarch with water to make a slurry. Whisk it into the hot cooking liquid.
Cover and cook on High for about 10–15 minutes, until the sauce thickens and turns glossy.
Combine and finish. Return the chicken to the crockpot and toss to coat in the sauce. Add red pepper flakes if you like a little heat. Taste and adjust seasoning—more soy for salt, honey for sweetness, or vinegar for brightness.
Serve. Spoon over rice with veggies.
Garnish with sesame seeds and green onions.