Make the teriyaki sauce: Add soy sauce, water, brown sugar, honey, garlic, rice vinegar, ginger, and sesame oil to a saucepan. Simmer over medium heat until the sugar dissolves.
Thicken the sauce: Mix cornstarch with cold water, then whisk it into the sauce. Cook for 1 to 3 minutes until glossy and lightly thickened. Let it cool, then reserve half for glazing.
Cut the chicken: Pat the chicken dry and cut it into even 1 to 1½-inch pieces so it cooks evenly.
Marinate the chicken: Toss the chicken with the cooled marinade. Cover and refrigerate for at least 30 minutes, or 2 to 4 hours for deeper flavor.
Thread the skewers: Add chicken to skewers, alternating with pineapple, bell peppers, and red onion if using. Leave a little space between pieces so they grill instead of steam.
Grill the skewers: Grill over medium to medium-high heat for 8 to 15 minutes, turning every 2 to 4 minutes. Brush with reserved sauce during the final 2 to 3 minutes.
Rest and serve: Cook until the chicken reaches 165°F, then rest for 3 to 5 minutes. Garnish with sesame seeds and green onions, and serve warm.
