Pre-heat the oven to 325 degrees. Before turning on the stove, mix together sugar, salt, ketchup, pineapple, soysauce, vinegar and corn starch. Mix until corn starch is dissolved.
Over medium-high heat, cook the barbecue sauce. Stirring continually, bring to a low boil just long enough to thicken.
Spray a 9 x 13 pan with cooking spray. Place thawed chicken on the bottom and sprinkle with chopped onion. Pour barbecue sauce on top, and cover the pan with tin foil.
Bake at 325 degrees for 1 to 1 1/2 hours or until chicken reaches an internal temperature of 165 degrees.
After chicken is finished baking, use kitchen shears or a metal spatula to cut into bite-size pieces. Serve atop rice or mashed potatoes.