Steps
1. Toss the diced chicken breast in cornstarch until well coated.
2. In a large skillet, heat vegetable oil over medium-high heat.
3. Add the coated chicken and cook until golden and crispy (about 5-7 minutes). Remove and set aside.
4. In the same skillet, add onion and garlic, sautéing until fragrant.
5. Add bell pepper and pineapple, cooking for another 2-3 minutes.
6. In a small bowl, mix soy sauce, ketchup, brown sugar, and vinegar. Pour this sauce over the vegetables and stir to combine.
7. Return the chicken to the skillet, tossing everything to coat well in the sauce.
8. Season with salt and pepper to taste, and cook for an additional 2 minutes until heated through.
9. Serve hot over rice or noodles.