Combine all ingredients for the sauce in a mixing bowl, whisk well, and set aside. (Don't add the pineapples, separate them from juice, and set aside.)
Cut chicken thighs into small pieces and cut off as much excess fat as possible in the process.
Dredge the chicken: Put cut chicken into a large zip-lock bag. Add cornstarch and some salt and pepper, close the bag, and give it some good shaking to make sure that all the chicken is coated evenly.
Preheat a large cooking pan first and add oil. Add dredged chicken and sear it on all the sides until almost done. Take chicken out and set is aside.
Add a little more oil if needed, and add in chopped onions, bell pepper, and scallions. Sauté for a few minutes, until softened.
Add chopped pineapple and cook together with the veggies until pineapple starts to get golden and soften as well. Take the vegetables out of the pan and set aside with the chicken.
Whisk the sauce again and add it to the same pan.
Slowly stir and let the sauce come to simmer and start to thicken. Once it starts to thicken, add back the chicken and vegetables.
Lower the heat and let the chicken finish cooking together with sauce.
Finish the dish off with some fresh diced scallions. Serve it over rice or simple roasted/sautéed vegetables.