Hawaiian huli-huli chicken is all about the marinade-slash-sauce, which was created as a shortcut version of homemade teriyaki sauce. Made with pineapple juice, soy sauce, brown sugar and ketchup, it's sweet, sour and packs a little kick of heat -- the perfect flavor combo for grilled meats. Because boneless, skinless chicken thighs cook in about half the time as the bone-in variety (the traditional choice), here we give the sauce a head start by reducing it on the stovetop before using it to baste the chicken.
Whisk together the pineapple juice, soy sauce, ketchup, brown sugar, red pepper flakes, garlic, ginger and 1/2 cup water in a large bowl or baking dish to combine. Season with black pepper.
Remove 1 1/2 cups of the marinade and add to a small saucepan. Bring to a simmer over medium heat and cook until slightly thickened, 6 to 9 minutes. Transfer to a small bowl and let cool, then cover and refrigerate until ready to grill.
Place the chicken thighs in the remaining marinade in the large bowl, turning to coat. Cover and refrigerate for 4 hours.
Prepare a grill or grill pan for medium-high heat. Lightly oil the hot grill grates with vegetable oil. Remove the chicken thighs from the marinade, letting the excess drip back into the bowl, place on the grill (see Cook's Note) and season with salt and pepper. Grill, basting with the sauce frequently, until the chicken is charred and registers 165 degrees F on an instant-read thermometer, 5 to 8 minutes per side. The sauce should caramelize and turn a deep red color. Transfer the chicken to a plate.
Drizzle the pineapple with the olive oil and rub to coat evenly. Grill until charred and lightly caramelized, 4 to 5 minutes per side.
Serve the chicken with the pineapple and garnish with the scallions.
If cooking the chicken thighs on a grill pan, you may need to do it in two batches to avoid overcrowding the pan.