Preheat the oven to 400 degrees F (200 degrees C). Lightly grease a baking dish or spray with cooking spray.
Pat chicken thighs dry with a paper towel, cut into 1-inch chunks, and add to the dish. Sprinkle with cornstarch, and toss until chicken is coated. Stir in onions, green and red bell peppers, and pineapple.
Add ketchup, rice vinegar, brown sugar, soy sauce, garlic, and ginger paste to a small bowl, whisk until well combined, and stir into the chicken mixture.
Bake until chicken is no longer pink at the center and juices run clear, 14 to 17 minutes, stirring halfway through. An instant-read thermometer inserted near the center should read 165 degrees F (74 degrees C). If sauce is a bit thick, stir in a splash of water to thin out until you reach your desired consistency.
Garnish with green onion slices and serve.
