Combine the sauce: In the crockpot, whisk together the reserved pineapple juice, soy sauce, brown sugar, ketchup, garlic, and ginger.
Add the chicken: Add the chicken thighs and turn to coat in the sauce.
Cook: Cover and cook on low for 5-6 hours or high for 3 hours, until the chicken reaches 165°F and shreds or slices easily.
Thicken the sauce: Remove the chicken to a plate. Whisk the cornstarch and water together, stir into the sauce, and cook on high, uncovered, for 5-10 minutes until glossy.
Finish: Return the chicken and pineapple chunks to the pot, toss to coat in the glaze, and serve topped with green onions.
Notes
No vinegar and no bell peppers, unlike the site's Sweet and Sour Chicken. This stays a purely sweet, mellow glaze on purpose.