Preheat your oven to 425°F (220°C).
In a large mixing bowl, whisk together soy sauce, pineapple juice, brown sugar, apple cider vinegar, minced garlic, and grated ginger to make the marinade.
Add the chicken thighs to the marinade, ensuring they are well coated. Let them marinate for at least 10 minutes while you prepare the vegetables.
On a large baking sheet, arrange the sliced bell peppers and red onion. Drizzle with olive oil, salt, and pepper, and toss to coat.
Place the marinated chicken thighs on top of the vegetables, skin side up.
Bake in the preheated oven for 25-30 minutes, or until the chicken reaches an internal temperature of 165°F (75°C) and is golden brown.
If desired, during the last 5 minutes of baking, you can sprinkle fresh pineapple over the chicken for added flavor.
Once done, remove from the oven and let the chicken rest for a few minutes. Then, garnish with sliced green onions before serving.