Pat chicken pieces dry and cut into uniform 1-inch pieces. Place in a medium bowl.

Whisk together soy sauce, brown sugar or honey, rice vinegar, sesame oil, minced garlic, and grated ginger to make teriyaki sauce.

Pour half the sauce over the chicken, toss to coat evenly, cover and marinate in refrigerator for at least 30 minutes (up to 4 hours for deeper flavor). Reserve remaining sauce.

Prepare pineapple by peeling, coring, and cutting into 1-inch chunks. Cut bell peppers similarly if using.

Heat a skillet or wok over medium-high heat with a bit of oil. Cook chicken in batches for 3-4 minutes per side until golden and cooked through. Remove and set aside.

Add pineapple and bell peppers to skillet. Cook 3-5 minutes until pineapple caramelizes and peppers are tender-crisp.

Return chicken to skillet with pineapple and peppers. Pour in remaining teriyaki sauce and bring to simmer.

Optional: Whisk cornstarch with cold water to make slurry. Slowly add to simmering sauce, stirring continuously until thickened, about 1-2 minutes.

Remove from heat. Garnish with sliced green onions and sesame seeds. Serve immediately over cooked white or brown rice.