Place chicken thighs in the crockpot and scatter pineapple chunks on top.
Whisk together reserved pineapple juice, soy sauce, brown sugar, rice vinegar, garlic, and ginger; pour over the chicken.
Cover and cook on low for 6 hours or high for 3.5 hours, until chicken is very tender.
Remove chicken to a platter and shred with two forks; return shredded chicken to the crockpot.
Stir cornstarch with cold water, add to crockpot, and cook on high 10–15 minutes to thicken.
Stir in sesame oil if using, taste and adjust seasoning, then serve over rice or noodles with sesame seeds and green onions.