In a large mixing bowl, combine soy sauce, brown sugar, ketchup, pineapple juice, minced garlic, grated ginger, sesame oil, and black pepper. Whisk vigorously until the brown sugar dissolves.

Add the boneless, skinless chicken thighs into the marinade, ensuring they are coated. Cover and refrigerate for at least 30 minutes.

Preheat your grill to medium-high heat (about 375°F to 400°F).

Once the grill is hot, place the marinated chicken thighs on the grill. Cook for 6-7 minutes on each side until the internal temperature reaches 165°F.

In a separate skillet, heat a tablespoon of oil over medium heat. Add mixed vegetables and sauté for 5-7 minutes until tender.

Prepare your plates by placing a scoop of cooked rice at the bottom. Slice the grilled chicken thighs and arrange them atop the rice. Spoon sautéed vegetables over the chicken and sprinkle with chopped green onions.