Prep the chicken: Trim excess fat if using thighs. Leave breasts or thighs whole; no dicing needed.
Make the sauce right in the crockpot: Add soy sauce, brown sugar, rice vinegar, sesame oil, garlic, ginger, and mirin if using.
Stir to combine.
Add the chicken: Nestle the pieces into the sauce. Spoon some over the top to coat.
Cook low and slow: Cover and cook on Low for 4 to 5 hours or High for 2.5 to 3 hours, until the chicken is tender and easily pulls apart.
Shred or slice: Transfer chicken to a plate. Shred with two forks or slice into strips, then set aside.
Thicken the sauce: Mix cornstarch with water to make a slurry.
Whisk it into the hot sauce in the crockpot. Cover and cook on High for 10 to 15 minutes until glossy and slightly thick.
Combine: Return the chicken to the crockpot and toss to coat in the sauce.
Serve: Spoon over warm rice, top with green onions and sesame seeds, and add steamed veggies on the side.