Mix the sauce: In a bowl, whisk soy sauce, honey, rice vinegar, mirin, garlic, ginger, chicken broth, sesame oil, and sriracha if using.
Add to crockpot: Place chicken in the slow cooker. Pour the sauce over the top, turning the pieces to coat.
Cook: Cover and cook on Low for 4–5 hours or High for 2.5–3 hours, until the chicken is tender and easily shreds.
Thicken the sauce: Transfer about 1/2 cup of hot cooking liquid to a bowl. Stir in cornstarch and water until smooth.
Pour back into the crockpot, stir, and cook on High for 10–15 minutes until the sauce thickens and turns glossy.
Shred and toss: Shred chicken with two forks directly in the crockpot, then stir to coat with the thickened sauce.
Serve: Spoon over hot rice, sprinkle with sesame seeds and green onions, and add steamed veggies on the side.