In a large bowl, stir together the pineapple tidbits, chicken stock, soy sauce, sesame oil, brown sugar, cornstarch, and garlic powder until well combined. Set aside.
Season both sides of the chicken thighs with salt and pepper.
Place an ovenproof skillet over medium heat. Then add the oil, onion, and chicken thighs once the skillet is hot.
Sear the chicken thighs for about 3-4 minutes on each side, or until the chicken has a golden brown color. (see note)
Pour the pineapple sauce over the chicken thighs. Then bake for 20-25 minutes, or until chicken is cooked through and sauce is bubbly and thickened.
Remove the skillet from the oven and allow the chicken to sit 5 minutes before serving. The sauce will thicken up as it cools.
Serve warm with chopped green onions and parsley on top, if desired.
Notes
If you don't have an ovenproof skillet, transfer the chicken and onions to a baking dish before pouring the sauce on top. Then bake as directed.