In a mixing bowl, I whisk together the pineapple juice, soy sauce, ketchup, brown sugar, ginger, garlic, apple cider vinegar, sesame oil, and black pepper until the sugar fully dissolves.

I set aside about ¾ cup of the marinade in a separate container for basting later, then refrigerate it.

I place the chicken thighs into a zip-top bag or container and pour the remaining marinade over them. I make sure everything is well coated, cover, and refrigerate for at least 4 hours or overnight.

When ready to cook, I preheat the grill to medium-high heat and lightly oil the grates. I let the chicken sit at room temperature for about 20 minutes.

I grill the chicken for about 5–6 minutes on the first side, flip, and cook for another 3 minutes. Then I start basting with the reserved marinade, flipping and basting every minute until glossy and cooked through.

Once the chicken reaches 165°F internally, I remove it from the grill, loosely cover with foil, and let it rest for 5 minutes.

If using pineapple slices, I grill them for about 3 minutes per side until lightly caramelized, then serve with the chicken and garnish with green onions.