Preheat your oven to 400 degrees Fahrenheit (200 degrees Celsius). Line a large rimmed baking sheet with parchment paper for easier cleanup.
In a medium bowl, whisk together the teriyaki sauce, soy sauce, ginger, minced garlic, brown sugar, rice vinegar, and the reserved pineapple juice. This is your glaze.
Place the chicken thighs in a large bowl or zip-top bag. Pour half of the teriyaki glaze over the chicken, tossing to coat thoroughly. Let the chicken marinate for at least 15 minutes while you prepare the vegetables. Reserve the remaining half of the glaze for basting later.
Toss the pineapple chunks, red bell pepper pieces, and red onion wedges with the vegetable oil on the prepared baking sheet. Spread them into a single layer.
Arrange the marinated chicken thighs among the vegetables on the sheet pan, ensuring they are not overly crowded.
Bake for 15 minutes.
Remove the pan from the oven. Brush the chicken thighs generously with half of the reserved, un-used teriyaki glaze.
Return the pan to the oven and bake for another 10 to 15 minutes, or until the chicken reaches an internal temperature of 165 degrees Fahrenheit (74 degrees Celsius) and the vegetables are tender.
Brush the chicken one last time with any remaining glaze during the last 5 minutes of cooking.
Remove from the oven. Garnish the chicken and vegetables with sesame seeds and sliced green onions before serving.
Notes
If you prefer bone-in, skin-on thighs, increase the total cooking time by 10 to 15 minutes and check the internal temperature carefully.
Serve this dish over cooked white or brown rice to soak up the extra pan juices.
For a slightly caramelized finish, switch the oven to broil for the final 1 to 2 minutes, watching closely to prevent burning the sauce.