Make Marinade:
In a saucepan over medium heat, combine pineapple juice, soy sauce, brown sugar, ketchup, vinegar, garlic, and ginger. Bring to a simmer. Stir in cornstarch slurry and cook until thickened. Let cool.

Marinate Chicken:
Place chicken in a zip-top bag. Pour half of the cooled sauce over the chicken. Marinate in the refrigerator for at least 1 hour (up to overnight). Reserve the remaining sauce for basting and serving.

Grill Chicken:
Preheat grill to medium-high heat. Oil the grates lightly. Remove chicken from marinade and grill for 5–7 minutes per side, or until cooked through (internal temp 165°F/74°C).

Grill Pineapple:
Brush pineapple rings with olive oil and grill for 2–3 minutes per side, until charred and caramelized.

Serve:
Plate grilled chicken with pineapple rings. Drizzle with reserved sauce and garnish with green onions.