In a bowl, whisk together the pineapple rings and juice, soy sauce, sesame oil, garlic, ginger, brown sugar, ketchup, tomato paste, red pepper flakes, nutmeg, and chili powder. Set it aside.
Pat the chicken dry and place it in a large ziplock bag or bowl. Pour half of the marinade over the chicken, making sure it’s well-coated. Cover and refrigerate for at least 20 minutes, or up to 24 hours. Keep the remaining marinade in the fridge for later.
When you're ready to cook, heat your grill to medium. Clean and oil the grill grates to prevent sticking.
Pour the leftover marinade into a small saucepan and bring it to a simmer over medium heat. Let it cook for 5-10 minutes, or until it thickens slightly.
Remove the chicken from the marinade (discard the used marinade). Place the chicken on the grill and cook for about 3 minutes on each side, turning until it reaches an internal temperature of 165ºF. During the last few minutes of cooking, brush the chicken with the simmered marinade and add pineapple rings to the grill for about 4 minutes.
Let the chicken rest for about 5 minutes before serving. Serve it over salads, rice, or enjoy it on its own with your favorite sides. Garnish with fresh cilantro and green onions.