In a large skillet, heat the vegetable oil over medium-high heat. Add the chicken pieces and cook until browned and cooked through, about 5-7 minutes. Remove the chicken from the skillet and set aside.

In the same skillet, add the chopped onion and bell pepper. Sauté for about 3-4 minutes until they start to soften. Add the minced garlic and cook for an additional 1 minute until fragrant.

In a small bowl, whisk together the soy sauce, honey, rice vinegar, cornstarch, black pepper, and red pepper flakes (if using). Pour this mixture into the skillet with the vegetables.

Add the diced pineapple and cooked chicken back into the skillet. Stir well to combine and cook for another 2-3 minutes until the sauce thickens and everything is heated through.

Serve the pineapple chicken over cooked white rice and garnish with chopped green onions.