Combine the soy sauce, brown sugar, pineapple juice, garlic, ginger, rice vinegar, salt, pepper, and sesame oil and seeds, if using. Whisk well, then transfer 1/2 cup to a large ziploc bag to use as a marinade.
1/2 cup soy sauce, 1/2 cup brown sugar, 1/2 cup pineapple juice, 2 cloves garlic, 1/4 teaspoon ginger, 1 Tablespoon rice vinegar, 1/2 teaspoon kosher salt, 1/4 teaspoon pepper, 1/2 teaspoon sesame oil, 1 Tablespoon sesame seeds
Pour the remaining sauce into a small saucepan and heat over medium-high heat until it begins to boil. Combine the cornstarch and water in a small bowl, mixing with a fork to create a slurry, then pour into the boiling sauce, stirring and cooking for about 1 minute until thickened. Set aside to cool until ready to use.
1 Tablespoon cornstarch, 2 Tablespoons cold water
Add chicken to the bag containing the reserved marinade and let it sit in the fridge for at least 30 minutes.
2 pounds boneless skinless chicken breasts
Preheat grill to medium-high heat. Thread chicken, peppers, onion, and pineapple onto soaked skewers, then grill over direct heat for 8 to 10 minutes, turning partway through to grill evenly on both sides, until chicken is cooked through and reaches 165°F when tested with an instant read meat thermometer.
1 red bell pepper, 1 yellow bell pepper, 1 green bell pepper, 1 red onion, 2 cups fresh pineapple, Metal or wooden skewers
Brush with the thickened teriyaki sauce and serve with any remaining sauce for dipping.