In a small bowl, mix the soy sauce, brown sugar, and cornstarch together until well combined. Set aside.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the chopped onion and sliced bell pepper; sauté for about 3-4 minutes until they begin to soften.
Add the minced garlic and cook for another minute, stirring constantly to prevent burning.
Push the vegetables to the side of the skillet and add the remaining 2 tablespoons of vegetable oil. Add the chicken pieces to the center and season with salt and pepper. Cook for 6-7 minutes, stirring occasionally until the chicken is browned and cooked through.
Once the chicken is fully cooked, add the pineapple chunks to the skillet. Pour the sauce mixture over the chicken and pineapple, stirring to combine. Allow it to cook for another 5-7 minutes until the sauce thickens and coats the chicken properly.
While it’s cooking, prepare the rice according to package instructions.
Once the sauce has thickened and the chicken is well coated, serve the sweet and savory chicken over a bed of fluffy rice.
Garnish with sliced green onions and serve immediately.