Cut the chicken into bite-sized pieces and season with salt and pepper. Coat lightly with cornstarch.

Heat sesame oil in a large skillet over medium-high heat. Add the chicken and cook until golden brown on all sides, approximately 5-7 minutes.

Add minced garlic and chopped bell peppers to the skillet; sauté for another 2-3 minutes until fragrant.

In a bowl, whisk together soy sauce, ketchup, water, rice vinegar, brown sugar, and additional cornstarch if desired. Pour the mixture over the chicken in the skillet.

Gently fold in pineapple chunks and let everything simmer on medium-low heat for about 5 minutes until the sauce thickens.

Serve hot over steamed rice or vegetables, garnished with sesame seeds or green onions if desired.