Begin by cutting the boneless chicken breasts into bite-sized pieces and coat with cornstarch. Let sit for a few minutes.

Heat a tablespoon of vegetable oil in a large skillet over medium-high heat. Add the coated chicken and cook for 5-7 minutes until golden brown. Remove chicken and set aside.

In the same skillet, add more oil if needed and sauté onions, bell peppers, and minced garlic for 3-4 minutes until fragrant.

Combine ketchup, brown sugar, apple cider vinegar, and soy sauce in a bowl, whisk until smooth.

Return chicken to the skillet, pour the sauce over, and simmer for 2-3 minutes, allowing flavors to meld.

Fold in pineapple chunks and cook for another minute to warm through.

Remove from heat, garnish with sesame seeds and green onions, and serve immediately.