Preheat oven to 325° F. Spray a 8x11-inch casserole dish with nonstick cooking spray.
Slice chicken breasts into 1-inch pieces.
Place ⅓ cup corn starch into a gallon size Ziploc® bag. Add chicken pieces into the bag, seal and shake the bag a few times to evenly coat chicken.
Add olive oil to a large skillet or wok over medium-high heat.
Once oil is hot, add chicken in a single layer.
Cook chicken for 1-2 minutes. Turn chicken over and cook for an additional 1-2 minutes until chicken is nicely golden brown on both sides, but not cooked all the way through. After browning remove chicken from heat.
Drain pineapple and reserve juice to be used in sauce in next step.
In a small bowl, whisk together pineapple juice (should be about ½ cup), ketchup, rice vinegar, soy sauce, garlic, stevia, crushed red pepper flakes and remaining 2 tsp corn starch.
Place browned chicken pieces in the bottom of the prepared baking dish. Add pineapple chunks, red and yellow bell peppers and onions. Evenly pour sweet and sour mixture over ingredients.
Cover loosely with foil and bake for 45 minutes until sauce is bubbly and chicken is cooked through. Turn chicken mixture halfway through baking to evenly coat.
Sprinkle top with green onions just before serving.